I feel like this is going to be an unpopular opinion, but... really, I don't see a problem with these people having this event, and I think the person who got up to sing the pro-LGBTQ+ song was in the wrong here. But let me explain before you crucify me.
It's pride month, and they have anti-LGBTQ+ views. Holding their own event for people with similar views is by far preferable to going and causing problems at pride events, and while I don't agree with their views, I support their freedom to hold them, given they aren't causing problems for others, which it sounds like they weren't.
An LGBTQ+ person "crashing" their event isn't really any different from an anti-LGBTQ+ person crashing a pride event, which I'd obviously have a problem with, so I think it'd be somewhat hypocritical to applaud someone for doing it to these people, however much I disagree with their viewpoint.
Edit: Yep, knew it'd be unpopular. Rather than just downvoting me, though, why not tell me why you disagree?