Assuming the adult(child) is not contributing financially to living expenses, the child has no right to an internet connection, electricity, room & board, usage of any equipment that they do not own, etc. So while the parent has no right to tell their adult child what to do with their time, the adult child has no right to free resources.
If you want to go down the "no right" path...
Assuming the adult(child) is not contributing financially to living expenses, the child has no right to an internet connection, electricity, room & board, usage of any equipment that they do not own, etc. So while the parent has no right to tell their adult child what to do with their time, the adult child has no right to free resources.