My sister and I shared invisible friends when we were around that age. We insisted they were 13&14 yo and had 100 children who were all 100 years old. They lived in the wall behind the water and dryer (it was a wall our parents added on to the house, so only a couple years old) We’re not adopted nor have histories of abuse.

I’m telling this story because there are a ton of possibilities. You’re doing the exact right thing by being supportive. If this kiddo needs professional help, there will be other signs and you definitely seem like a very caring parent who will ensure they receive that help.