AIBU to not understand why he didn't tell me about her?

What you do right now is support him. Now is not the time for deep questions but as you talk to him about her you will find answers

You do not accidentally forget to mention a terminally ill "best friend" for three full years.
Keeping someone that central hidden requires deliberate, daily omission. Her family contacted him immediately because he was clearly a primary person in her world, yet he erased her existence from yours. Whether physical or not, he maintained a secret emotional life.
Give him space to deal with the immediate shock, but do not doubt your instincts. A three-year lie of omission does not get erased simply because the secret ended in tragedy.

I would assume he was at least having an emotional affair with her.

Sorry, OP.

Take things in stages. Don’t do anything now, there will be a funeral which presumably he will attend. A potential issue is if you accompany him. Obviously you did not know her, but many people do attend funerals with their partner.
Then once that is all over you need to establish facts and evaluate. Why didn’t he tell you. How is your relationship otherwise?
In my experience secretiveness generally ( including about contact with ex’s) is a huge red flag.

@Shardonneigghhh Perhaps you acted oddly when he mentioned his ex previously so didn't mention her again?

I don't see the issue. It's his ex. So they didn't work out as a couple but remained in touch as friends. I'm friends with my ex.

He has just told you someone important in his life has died and you're making this about you??

Don’t be ridiculous. Of course it’s a slim possibility, but more likely they just kept in touch and he felt awkward telling the OP as it was an ex, and the OP might not like it.

Nah, sorry, I'm not buying it.

This would be relationship-ending for me. Who lies about something like that for three whole years?

Gosh that really puts you in a difficult position OP. Of course you can't delve into this when he has just received this news of her death. But honestly you must be totally floored by what can only be interpreted as his deliberate deception.

What ever his explanation of why he never mentioned his regular contact with his ex and that he never felt the need to confide in you about his concern for her health issues I don't see how you can trust him going forward. He has shown at the very least a tremendous capacity to compartmentalise his life. It must be very unnerving for you.

This isn’t just keeping in touch. This is someone who he refers to as his best friend, but never mentioned to OP. I’d think that was weird even if the best friend was a man who he had never dated!

I can imagine that he thought that it was awkward to mention that he considered his ex his best friend.

Maybe he didn’t see her as that until she passed away?

Does he consider your relationship to be serious?

Did she come to visit his home town?

In those 3 years of being together did he talk about any other relationship he may of had and just kept quiet about his ex?

Just because she died doesn't take away the fact that her partner was potentially was cheating on her??

Do they have shared children? If not, then I guess she is million exes back and their previous relationship is kinda irrelevant. They are just uni-days couple which isn't that serious.
My partner hasn't met and doesn't know anything about some of my friends and exes I am not hiding it from him, it just never came up. I am not holding them in my head all the time and don't think I need to inform my partner every time I receive a text.

What evidence do you have that he was "potentially cheating"? Zero from what we've heard so far.

When he says she was his best friend, does he not mean in the past, i.e. when they met and were dating? I am of an age where people who I was close to when I was younger have started dying, and I have felt very sad about them, also for some time afterwards, even though we haven't had contact for years. I have grieved exes who I have had sporadic contact with - it doesn't mean I had feelings for them.

It has to do with recognising the passing of time, thinking about things that meant a lot to you that are gone for good, etc. It doesn't mean that he still has feelings for her or has been in secret contact all this time.

The DC aren't with the ex that died then?

Was the separation amicable, if so there are many many couples that stay friends with their ex's once separated, maybe they just didn't suit being together.

If they haven't seen each other for 3 years the chances of him having been unfaithful with her are incredibly slim.

That'll be the 'kangaroo court' of MN sheep says so.

Like fuck would I be supporting my partner after his ex fuck buddy he’d been keeping a secret for 3 entire years. How has it ever been mentioned even in passing that they were still in touch, let alone she was literally his best friend??

Yes, but that's not the case here. The OP's partner was in regular touch with this woman for the past three years. He's devastated by her death because, as he says, she was his (current) best friend. He was also phoned by a close family member of the deceased to inform him of her death, so his number was obviously known to them or easily accessed from the woman's phone. So this isn't some long ago faded connection.

The fact it was a secret relationship doesn’t look great.

Whenever there are threads on here about male/female friendships, I’m always on the side of thinking these are totally fine. My best friend is a man! But he’s not a secret from DH.

Do you know what 'potentially' means? And no I haven't got my evidence just assumptions a d opinions which is allowed on mumsnet last time I checked

if someone reacts like that, no wonder their partner wouldn't mention their ex, ever!