So a bit of background, im living in a house with 3 other boys and I've been friends with them since the first year of university and this is the first time we've shared a house together. It's amazing, we all get on, do the same substances and we never have arguments and im planning to live with one next year when we do a master's. 
He (Steve) met his gf (Jess) last year when we were all on placement year and have been going out ever since, when he told me about her and I was excited to meet her as this was his second serious relationship and I wanted to meet her. My first impression of her was that she was polite and quiet (due to nerves I assume)
Then she came around a week later (she comes over every weekend without fail and has booked train tickers for the foreseeable future) this was when things started to go wrong. A friend of ours who lives in another house invited us over for a smoke sesh (weed) and we brought her along (we made sure she was comfortable with us smoking and told her she doesn't have to if she doesn't want to) she sat down with headphones on, didn't speak to anyone or pay attention to us, "fair enough" I thought "must be nerves". Boy was I wrong
45 minutes in she stands up and began screaming at Steve saying "I can't believe you aren't talking to me, this is so rude and inconsiderate" and goes off on one, then storms off slamming doors and yelling in the hallway and causes a huge scene, this went on  for about an hour. Everyone was shocked and didn't know what to think/do so we ignored her. They leave together and she ignores him the whole walk home. When I asked him what happened, he told me Jess claimed it was a "panic attack". (I understand mental health is complicated but this didn't seem like a panic attack) She also makes him call her every hour or so and send photos of who's he's with and where he is when he leaves the house, constantly accusing him of cheating and not caring about her 

This has happened every weekend she's been coming over and has been constantly arguing with him, telling him that he's stupid, causing drama on nights out and has just been rude to all of us. I've told her "im gonna respect your relationship but don't expect us to be friends" and have told him many times that I don't think she should come around every weekend as it's not healthy. It's gotten to the point now that when I get drunk and she's there I tell her what i think about her (like she's not good for him and she's controlling) and it's not getting better as i can hear everything she says in his room so I can hear her talking shit about us 

I understand she's a human being with issues but there's a limit I can take and I think I might be taking it too far now and it's now straining my friendship with Steve but I honestly can't take it anymore. 

Am I being an asshole? Should I let them have their relationship and focus on being his friend when she's not around? 