My friend found out that his ex recently came out as trans. As he was telling me about it, it led to a conversation where he realized he's not quite sure what to refer to him as. Basically, what he wasn't sure of is if his ex identified as female while they were dating but now identifies as male, does that make him his ex boyfriend, or would he still call him his ex girlfriend considering that he thought he had been dating a girl at the time? I'm usually the one my friends come to with LGBT related questions but this time we both ended up talking in circles and I wasn't really sure what to tell him.
Well there was another question like this that basically said to use their new pronouns, gender, name etc so it would be ex-boyfriend but it really depends on the person in question; I personally wouldn't mind either way.
Imo... Your friend could say "ex" or "ex partner". If it doesn't conflict with his sexuality, he could just say "ex-boyfriend". I guess if he's straight and says "ex-boyfriend" though people are going to question it, and then it might get to outing his ex, so in that case best stick to neutral "ex"?
I wouldn't complicate it much. When they were dating he considered him to be his girlfriend so why to change it now? It's like you had a colleague who was working as a secretary back then but now they are a teacher. Does it make them automatically a teacher in the past as well? It doesn't right? I agree though that just "ex" or "ex partner" could make it less complicated for everyone involved.
I suspect the ex would be a lot more comfortable being called an ex-boyfriend, and it's the respectful thing to do, but at the same time I understand how it feels a bit inaccurate given that your friend didn't really have a boyfriend at the time. I would recommend just saying "ex" or "ex-partner" like others have suggested because it's respectful to the trans guy without having to feel uncomfortable or inaccurate for your friend.
I agree with Jesse, Ex sounds neutral. Most people leave it at that, because generally talking about Ex's is uncomfortable.