There's plenty of ways to make games outside of Roblox. Maybe they could sit down together and work through some Löve2D or Godot tutorials? No one can take that away arbitrarily
“Sit down together” might be impractical here, if GP’s child’s friends are e.g. friends they made before a move, who are thus quite far away physically. Or friends with snobby parents who won’t let them come over to GP’s house for whatever dumb reason. Or friends with extra-curriculars such that their free time never lines up with GP’s kid’s free time—meaning that only async collaboration will work.
(That’s just a steelman position, though; in general I agree.)
This thread is like me complaining Google face-scan-gated Google Docs and people are saying “he can just sit down with the friend and learn LaTeX together!” Yeah, no.
Fwiw he does Godot too. It’s fun, but it’s purely solo. Godot’s answer to collaboration is Git, which is a complete non-starter for a 13yo. Note, I don’t judge them for it, they compete with Unity, not with Roblox.