To challenge a colleague's insensitive behaviour on my friend's behalf?

You cannot cross contaminate the underside of a pizza.
Unless the fishslice was in contact with the topping, there is no need for hysteria

Given that he was using a fish slice to remove the pizzas I'm struggling to see what the issue is as it wouldn't have come into contact with the topping. On the other hand, if he used a knife or pizza cutter to cut the meaty pizzas first, then the vegetarian ones then I agree that he would have been in the wrong.

However, I wouldn't have challenged him the way you did.

You made yourself look a complete fool @NightOwlGirl and unfortunately are probably now the laughing stock of the other department.

That an actual written complaint has been made about you may sound OTT but verbal abuse in a professional setting isn't really on.

Yeah, this. It's hard to see how "cross contamination" could have happened. You were unnecessarily belligerent and you should apologise.

What if "Emma" is a strict vegetarian for religious reasons? Hindus generally don't eat meat or fish and if so, cross contamination is a very real thing and people need to be more aware of it.

I'm veggie and HATE when other people make a fuss on my behalf. So embarrassing.

You could have had a quiet word with Emma but you chose to make a drama out of it. YABU

You don’t take a pizza out the oven by its toppings. You touch the base and remove it without touching the top.
you sound like you were spoiling for a fight. Unnecessarily aggressive and none of your business.

I mean, should he even have been using a FISH slice to remove the pizza from the oven...

The only person you should have spoken to was Emma.

The poor guy offered to cook for everyone then gets you coming at him obnoxiously fighting for an issue that you perceived.

If you quietly told Emma that he removed the pizza with the same utensil then she would have probably been fine.

You could have handled it much more gently. This guy wasn't a paid pizza server presumably.

He shouldn't have laughed it off but I expect he was nervous.

Poor understanding of how cross-contamination works there, OP
If Emma is that worried about meat anywhere near her food, she needs to deal with the clloking/reheating or whatever it is you do to food at such times.
You were v unreasonable for causing a scene and embarrassing the bloke, emma and yourself.

Poor guy was just trying to be nice

Sorry, you were way OTT and very rude.
The guy offered to make pizza for a team lunch. That's lovely. He is not a professional caterer working in a professional kitchen. Your friend knew this. As a strict vegetarian would she eat pizza with cheese on it anyway? I have one friend who is a strict vegetarian and she won't eat "normal" cheese. But she is the only strict vegetarian I know so I am completely ignorant about it.
In this situation, if someone has strict dietary restrictions, then I would expect them to bring something they can eat like any other working day and not expect a colleague to have the prior knowledge or experience to cater for them (I wouldn't have thought about it to be honest).
You embarrassed a colleague in a situation that was attempting to create a closer working relationship and then walked away ensuring that there would be no bridges built.
I don't think that you have done yourself or your friend (she will be guilty by association) any favours by acting like that.

I agree with @jacks11 I also think the OP
should apologise to the poor bloke helping out for embarrassing him and being an all round arse.

if your good friend Emma was so precious about using the same fish slice she would also have been precious about using the same oven as the one the meat feast was cooked in and she should have brought her own food. Does she not have a voice of her own anyway?

if it had been me I would have kept my beak out and just given Emma the heads up so she could make her own mind up.

I'm pretty sure that most vegetarians (I'm one) have eaten pizza, bbq halloumi etc at parties, festivals etc where there's been contact. We manage to share work fridges and microwaves, eat at buffets, share picnic foods without stressing about cross contamination.

You didn't need to be Emma's voice. I suspect she was a bit embarrassed by the whole thing.

I don't know a single vegetarian who would care about this.

If Emma had an allergy and thus actually needed to be careful of cross contamination she would have told him herself or (more likely) brought her own food along.

He was helping out by bunging a few pizzas in the office oven, not a professional chef. You embarrassed him in front of both teams, presumably. You behaved really badly, and possibly antagonised relations between the two teams.

I'm a vegetarian and would have been mortified if you acted like that on my behalf, way OTT.

You made it all about you and what you ''achieved'' yes I think you went OTT, if you did it to me I would have put a formal complain in about you and your behaviour

Yes you should have asked him about it but there was no need to raise your voice and be rude. The guy was just using his break to make lunch for everyone. Personally I would have asked him if there had been contamination, then quietly slipped off and got the vegetarian friend a takeaway lunch elsewhere and bought it back to the room for her to eat. I would not have made a fuss, the bloke wasn’t made aware beforehand she was strict vegetarian and he could simply be asked to use a separate spatula when he starts to cook pizza next time.

If someone is so strict, they wouldn't eat a pizza that had been cooked in the same oven anyway. Much more likely to have cross-contamination from the meat sizzling onto the veggie pizza than from sliding anything under the base (the bottom of the base is the least likely part of the pizza to touch meat!).

You were being uttely ridiculous, especially because this was not even your battle to fight, but Emma's, who didn't seem to be paying any special attention to the pizzas' preparation, which hints strongly towards her not giving a flying fuck about the matter.

Both pizzas would have the same base so I don't see how it's contaminating anything, unless anyone's allergic then refusing to eat it is incredibly precious and you were rude reacting like this.

If he complained about being called out on it then he has a lot to learn about diversity.

Yuck. A common sense response like; being easy-going, politely noting it for the persons future reference and then carrying on trauma/drama free, just eludes certain sectors of our current society.

It's an office, not a bloody restaurant.

What did you want him to do? It was already done 🤷‍♀️ I'm sure it was not intentional. All you did was embarrass him and make yourself look nasty and OTT.