I'll agree with everyone else here, but you should also decide for yourself how far you're willing to go. Bring it up to her but decide: "Am I willing to do this as a favor to her?" "Am I willing just just cut ties with the business?" "At what point is the friendship worth more than me being an artist here?"

I hope it doesn't get to that point, but they are a few things worth considering. Hopefully your friend will be receptive, but she's not, you have to figure out when you're willing to cut your loses.

Also, what exactly is your complaint with this other artist? When I've seen adoptable pet stuff it doesn't seem like it takes a lot of working side-by-side.