Son wouldn’t let me use his loo !

Being autistic is absolutely no excuse to deny someone to use your loo. No matter how uncomfortable you are, being a rude and horrible host has nothing to with ND. Loads of ND people can deal with the discomfort and allow someone basic amenities in their home (in this case she wasn't even expecting to stay and chat or even interact with the GF).

You can be autistic and also have basic human decency rather than expecting the entire world to revolve around your own needs.

I’m going to be quite uncharitable here based on unfortunate personal experience.
My very sweet BIL’s ex used anxiety and overwhelm as a way to be controlling, manipulative, and emotionally abusive. It wasn’t entirely her fault, she was brought up to be like that and her whole family is fucked up and deeply dysfunctional in one way or another.
Keep your DS close, make sure he knows you’re there to help him if he needs it. It took my poor BIL four years to fully extract himself from this terrible relationship and it nearly broke him. I know we’re always told to ‘be kind’ and ever so understanding, but there’s limits to that, a big one being a negative impact on other relationships and the wellbeing of yourself and your loved ones.
Picking up the pieces afterwards is pretty shit, I know, I’ve done it.

I'd have wee'd in the driveway stating 'when you gotta go you just gotta go.'

Where is your evidence that OP actually said that to her son ? She cleaned his bathroom and his kitchen. She says she does cleaning for him when she visits. That’s what mums do. Just because she thought it, doesn’t mean she actually said it and you’re inventing things OP hasn’t said to suit your own purposes.

I hate when someone just arrives, and I never do it at anyone else's house. But in this case it's not the girlfriend's house and it's reasonable to invite someone in on an arranged visit. The son just didn't handle it well at all and I don't believe it was anything to do with his girlfriend anyway.

I don't think anyone has said only one thing at a time can be rude.

I think even the most unworldly people would realise, unprompted, that a relative visiting from a couple of hundred miles away would need the absolute minimum in hospitality, when the arrangement is to meet at the house. If the son had offered to meet her in town or whatever (which he probably should've done knowing he couldn't invite his mum in), that's a bit different and the expectation wouldn't be there.

So is being insistent (and calling someone’s house disgusting is fairly bad manners)

Can’t get my head around thinking that I can never move on from something like this with my own son!!

So there was clearly a reason. If it’s a one off then the OP needs to accept that. He didn’t want her to come in the house.

I don't give a shite, it's rude.

It’s his home. No reason should be needed. If someone said my house was disgusting then they wouldn’t be walking through the door again

But he regularly has her over to visit, even when he has friends around, and this is the only time it's been an issue, which implies that she's not.

The reason he gave and the other reasons out forward are bullshit. The OP went to visit her son, it was her only reason for being there. There is no reason to not let her use the toilet, none. The girlfriend can have all the mental health issues in the world and it still doesn't justify it.

Your son letting you clean for him when it suits him and then not letting you use his toilet is just fucking rude.

Wow, that sounds like a great way for a parent to wreck their relationship with their adult child.

If either of my parents had told my 23-year old self that it was "unacceptable" for me not to let them into my home for any reason, I'd've told them "my house, my rules" and that if they didn't like it, they could get to fuck. (I come from a long line of sweary, stroppy fuckers, so this would not have shocked them in the slightest.)

Thankfully, my parents weren't judgmental twats so it was never an issue, but it was for a friend whose parents were shockingly judgy, to the point that he was NC with them for the best part of 30 years.

Exactly.

Mums often spend years complaining about cleaning up after teenage boys until fly the nest.

I'd imagine, unless OP really gets a kick out of cleaning piss and pube covered toilets, that for her to feel the need to start scrubbing her son's bathroom now that he's left home it must have been genuinely disgusting. And if so, telling him as much isn't being snarky or judgemental.

I know all about anxiety and depression I lived with my mother for 16 years.

It never got better. It only got worse. The more and more ridiculous things we had to do to keep the peace and not set her off. My life wasn’t my own. I couldn’t make a choice it was her way or the highway. The screaming fits when I chose the highway
Sometimes I wonder if someone had pointed out how ridiculous and selfish she was being instead of pandering to her behaviour in the first place things would have turned out better (they couldn’t have got worse)

The more we were made to cave to her demands the more nasty and selfish behaviour we got if we dared to say no.

Diagnosed by doctors and psychiatrists

Or that his GF has severe MH issues.

And agreed with it, if it was correct.

I would be more concerned that he didn’t feel comfortable letting the OP in, which implies he feels she’s intrusive

House was probably a shit tip!!

Something tells me there's more to it than that, maybe they smoke weed or other drug paraphernalia in the house and didn't want you to witness. Otherwise GF could've left for a walk to let you use the bathroom. Either way, shocking!

Even if she exists and has anxiety then why assume the OP would be going into the house?

He’s 23, he’s a grown man not a kid. I would be very concerned if my adult son had either a girlfriend who is so mentally unwell she couldn’t tolerate me going into his home for a wee, and/or a bathroom that was so disgusting he was ashamed to let me use it. Completely abnormal and very worrying.

I know his home is sometimes a mess, on my last visit I throughly cleaned the kitchen and bathroom. It was disgusting.

There you go, apparently it was disgusting. Don’t you think he would have been aware of her opinion?