Hi, and welcome!

I realise this is all very new and confusing to you, and it's clear from your posts that your heart is in the right place and you want to do what's best for your crush. You sound puzzled about why he's dating a lesbian when he claims to be a guy, right? Shouldn't he be dating straight girls if he's a guy? Or if he's dating a lesbian, does that mean he's not really a guy? Is that what's confusing you?
If so, let me try to explain it to you: about 1/3 of trans people are straight/heterosexual, i.e. attracted to the opposite gender. Another 1/3 are bisexual or pansexual. Your crush sounds like he is either straight or bi/pan: whatever he may currently look like, he is in actual fact a guy who is attracted to women. But when it comes to dating, the problem he has is that
other people have always seen him as being female. So if he's been trying to live as a woman and he's sexually attracted to women, who is available for him to date? That's right: women who are interested in dating women, i.e. lesbians or bisexual/pansexual women.
That's why he's dating a lesbian. We're all human and we all need love, so he has to find it where he can. The only women who were interested in him were women who like women, so that's the group he had to pick from. Dating a lesbian doesn't change the truth of who he is in the slightest - and dating him doesn't change
their identities either.
But as he continues through his transition and starts looking, sounding, feeling and (god help him) smelling more masculine, the sort of women who are attracted to him will change. Most (but not all!) lesbians will lose interest in him and he may start to feel uncomfortable in lesbian spaces; bisexual & pansexual women may still be interested; and those straight ladies are going to start taking an interest in him! At that point, he'll have the option of dating a straight woman if he wants.
But do be respectful of his relationship with his girlfriend, just as you would with any other relationship. If people are in a committed relationship, that relationship should be respected - especially by their friends. And also: be aware that there are plenty of women who identify as lesbians who are in happy, committed, long-term relationships with trans guys. In many cases, they fell in love before he transitioned, and that love has kept them together. It's up to each couple whether they stay together through his transition, but if his girlfriend is warning you to stop sniffing around her man, she probably loves him a great deal and is prepared to at least try to keep their relationship going through his transition. They've already been together for two years: this isn't a casual fling!
So if you truly want to be there for him, the first thing you should do is to ask your friend what pronouns he prefers. Then use those pronouns at all times, including in the lesbian group if that's what he wants. It'll help get everyone else into the habit of addressing your friend correctly. Tell him you'll be there for him and will support him - but keep a respectful distance when it comes to romantic matters.
BTW, what T.K.G.W. was trying to say is this: in the trans community, it's considered inappropriate to call someone by the wrong pronouns. If (as you say) your crush has told you that they're really a guy and are about to start transitioning to male... then your crush
is a guy the respectful thing to do would be to refer to him with male pronouns unless he specifies otherwise. For the overwhelming majority of trans guys, being called 'she' or 'her' is excruciatingly painful and very disrespectful. We've all experienced it at some point of our lives, so seeing another trans guy being referred to as 'she' or 'her' brings back bad memories of what it felt like for us. Please don't do it - especially not in capital letters (that's what transphobes do; please don't be like them!).