I'm going to try and be brief, so happy to answer any questions.
To give some background...
I have been with my partner for 2 and a half years and recently got engaged.
He was married for around 20 years and they divorced 12/13 years ago.
Following that, he was in a relationship for several years which ended before he and I got together. We were friends (as part of a social group) for a few years before we got together and I knew his most recent ex.
He is amicable with his ex wife and they have adult children together.
He plays in a band. I used to go and see his band play and his ex wife didn't go to any gigs but since we have got together, she turns up to nearly of them. Despite having a partner, she has made it very clear that she would like to get back with him. This is something she never did when he was with his ex although he was aware that she would have reconciled if he'd wanted to.
I'm not worried that he is interested in any way, but my issue is this.
When she comes to gigs, she behaves in ways that she used to when they first split up. She dances in front of him whilst he is on stage and openly videos him. Just him - not the band. She has her phone upright (rather than landscape to catch the whole band) and is standing so close to him in front that she will only be filming him.
I think this is a bit odd. My exh has a performance based hobby and I would have no interest/think it a bit inappropriate, to go and see him. I wouldn't be standing in front of him filing him.
Anyway, all of this is a bit odd but what I really don't like is that she stands there staring aggressively at me. I did speak to her the first time I met her and she was absolutely fine with me. Their kids were there too and I thought it was odd that she went but she was with the kids. She was perfectly fine with me but did speak to me as though she had 'ownership' of him and was 'allowing' us to be together.
Now, she turns up without the kids and does the same - dancing and videoing him but just standing there staring at me stony faced. I've tried smiling at her, tried talking to her again and she just blanks me whilst staring at me. On a couple of occasions she's been stood on her own 6-10 feet away from me just standing and starting at me.
He doesn't have her as a friend on SM but his profile is open. He blocked her last summer because she posted a comment on one of his posts when we were on holiday last year. It wasn't a friendly, "Looks great, have a lovely time," comment to us both but more 'territory marking'. He didn't like it and blocked her.
He wants me to go to his gigs. I like going to them, but I really don't enjoy this open hostility. Being watched all evening with someone standing 6 feet away staring at me isn't how I want to spend my Saturday nights if I'm honest with you.
She tends to only turn up to gigs at a couple of venues but they play at them quite a lot. Would it be unreasonable of me to to just not go to these gigs? Of course, this doesn't take its account the times she might turn up at other venues.
He wants me to go and because he doesn't see this behaviour, he doesn't really get how unpleasant it is.