After Joey returns from the void, he discovers that Gavin is doing something hideous to Rosa. Joey can't do anything to Gavin or stop what he's doing; only Rosa can stop it.
You can't switch back to Rosa right now. The only thing Joey can do is talk to Rosa and somehow convince her fight Gavin's mind control.
Rosa is completely in Gavin's power, like a hypnosis subject in a cheap 1960s sitcom. Being gentle and soothing isn't going to help.
You need the most obnoxious, cage-rattling, offensive remarks you can find -- the verbal equivalent of a slap across the face -- to wake her out of this trance. When you pick the right response, Rosa will turn away from Gavin just a bit.
Remind her of Gavin's four victims that you've met, then pick the nastiest, most personally insulting remarks about each one. Specifically, choose these responses: