OK, this is a pretty shitty situation and I do think the ASPCA could have done more. But did they call around to all the local rescues and shelters to see if the cat turned up I would think if they did, they might have been able to find the cat before it was rehomed. I'm also assuming the cat escaped out the door and wasn't let outside intentionally.

But yes, AsssumiIng (again) your friends want the cat back, they should follow up with an attorney and find out what their options are. Pets are property so they probably have some recourse. Have they been able to reach out to the people who adopted the cat? I know that if I sound out I accidentally adopted someone else's pet, I'd give it back.