This story sounds very familiar to me, I feel like I've read it somewhere, but I don't see any other posts from you about it. Well, I guess it is something many poly people face at some point -- when one is at odds with an ideology or practice of a metamour whose influence on one's lover seems threatening or frightening in some way.
I think I would be feeling similarly concerned. I like a few kinky things in the bedroom only, so anyone who likes 24/7 domming freaks me out a little, too. From what I've read (I have a bunch of ebooks on BDSM that have been very illuminating), being a dom/me is supposed to focus on the benefits and pleasure it can give the sub, but I have also heard that it's difficult to find a good dom/me. The problem is how do you know someone is good at it or not until after the damage is already done? I guess people can ask for references?
As far as the play party, I wouldn't be too worried about his going there. My understanding about those things is that they are not total free-for-alls, and people who organize them consider everyone's safety and consent to be of tantamount importance. There are people here who can tell you about these things from experience (which I don't have), but I believe just watching is okay at most play parties. Also, I do know that in NYC, the big BDSM organization holds introductory meetings for newbies run by volunteers. They meet at a coffee shop, talk, answer questions, and then take any interested newbies to a dungeon for a tour. Everything is respectful (you can read about that here: http://www.tes.org/novice/
). Maybe he has something similar where he lives, and can learn about D/s from workshops without actually being dommed (unless/until he wants to).
I think that there are a few ways for you to personally deal with this. One is to trust his judgment and his word. I know that some people feel like their partners are bad at making decisions and therefore they cannot be trusted, but if your bf is adept at respecting boundaries, expressing when he is uncomfortable with something, and has a brain in his head, give the guy the benefit of the doubt that he knows what he's doing and will keep his word to not go farther than he's said he will with her.
Another thing to do, if it looks like they are getting closer, is to communicate with her directly and express your fears and concerns to her. Tell her that you have been burned before by a lover's domme, that even though it's long-distance and you only see him every few months, your bf holds a special place in your life and you care deeply about how he will be treated, and that you want to know she will respect him and his relationship with you. Set aside your disgust to talk to her and see that she is another person with feelings, hopes, and dreams, too.
Finally, you could ask yourself if this is a deal-breaker for you. If it is, then let him know that if he does start a sexual relationship with her, that is something you cannot accept or live with and you would end your relationship with him. Not as an ultimatum to manipulate him, but as a choice you are making because you cannot abide by his involvement in something that creeps you out. And then stick to your guns. Maybe you will have to walk away.