I don't think Beth's wanting to work for Petofi is that out-of-the-blue...she's angry and she's hurt over Quentin throwing her over for Angelique. The angry part of her wants to hurt Quentin as badly as he hurt her. She knows what a pain in the neck Petofi can to Quentin, so she aligns herself with him.
I just posted on this elsewhere. It's like Tim and his revenge plan. If what's most precious to you is pulled out from under you, you tend to snap, and say, the bad guys have it all, I'm in. Then eventually you realize it's just not you, and can't be a bastard after all.
Unless you can. The real bastards must come from somewhere...