Same reason we instantly teamed up with Cirina to help her rule the Steppe. Teaming up with her was recommended by someone we trust. We know and trust Erenville, and he vouches for her (least for her good intentions).

I'm expecting a good character arc for her. My guess is she's someone who didn't really want the throne, but sees the other challengers as threats to the way of life she loves. We know one of the Mamool Ja is a warmonger. We don't know much about the other. Green G'raha seems to be a big tech bro, and they're always bad news. Not to mention it seems like he probably recruited Thancred and Urianger, and one of those betrays us every expansion, so screw team-catboy...