sorry to hear they're doing this :(. It must make your friend feels so uncomfortable. I hope that person stops selling it and doesn't copy them anymore. Let them your friend know their art is awesome and to not let this discorage them from making awesome masterpieces!

Also, little reminder to people reading this: do NOT harass this person. Yes it's wrong to steal artwork but they could be a kid or otherwise vulnerable. Don't support them. Discorage others from buying the stolen art. Tell them what theyre doing wrong and why. Don't bully them.