Be honest, but give them ideas on how to make it better. Also, don’t say things like “that’s awful” but phrase it better like “I am not sure the general public would appreciate your art.”
I recently had a situation where someone asked me to proofread something. I gave them honest feedback and they were really annoyed. All they were looking for was fake praise. So, if you think this is what they are looking for, just go with that.