Only if I hadn't faced my own homophobic issues. I have found in many situations like his, one can make it a win-win situation.

A question I have that you didn't touch on here is how it affected his date. How did they interact while this was going on and after?

It depends on the situation, now if the woman coming onto your friend knows that your friend is in a new relationship and the new boyfriend sensed her sort of messing with him by continuing her pursuit, I can see where he would feel disrespected.
If the relationship is new, not much trust has been built up yet and so therefore here they are, out and about and he doesn't know how his Lady is going to react to things or whether those two had history or not. I am not sure it was about it being a lesbian, I bet even if it was a guy he would feel a little disrespected.

They were together, they were out, unless there had been some talk of maybe having a threesome this other person is not respecting that they were together that night.
Maybe I am a jerk, or insecure but it seems from my perspective that the other woman was purposely testing a boundary. Or maybe your friend AND the other woman were testing a boundary with the guy.

I see people as people, whether they are straight or gay, someone coming onto someone is someone coming onto someone, gay straight bi or whatever. If I am on a date with someone and someone else is relentlessly trying to "horn" in on it I would feel uncomfortable whether I was playing the part of your friend Lady or the irked boyfriend.
If it's new, he doesn't know that she would or wouldn't do anything about the flirting.

Sounds like plain rudeness, genderless no matter what their preference. Once is fine, cool. The second time you're turned down, they meant it, they're not interested. After that, fuck off, you're just being a dick, get used to disappointment.

Was there no possibility for a threesome? Sounds like a opportunity to me.

I am way too lazy to read all 30 posts...I read about 5 into the thread...lol

I am not sure it is about gay or straight. Maybe just about plain Respect in her boyfriend's eyes.

I use to go out with my gay guy roommate all the time. When straight men came on to me, he got really upset. His thoughts were...
How did these men not know we were together? Trying to pick up someone when they are with their man or woman, right in front of the person, well is disrespectful.... it doesn't matter if your male, female, straght, gay or bisexual... it's one's intentions.

I was in a club once and this woman was trying to come on to me. Before I could respond, my husband came over to me and she got visibly offended! It seemed she felt he was muscling in on her game or something. It never occured to her that he and I came in together.