I think that after sammeal was killed she really had no one on her side, now that wouldn't be so bad but she also knows the other forsaken have joined together in some way or another. she was singled out by moridin to watch him take Demandred and co. down a peg, so knowing moridin had 2 forsaken with him and then Demandred and co., I see no reason she wouldn't take in a partner, especially one she thought she could control because of the homelessness. Of course I haven't read the prologue so I might be missing something, but it would make sense to pick up some help, no matter how dubious, quick before the last battle comes and because she is really outnumbered and out gunned