Most kids stop using a potty chair once they are big enough to use the regular toilet.
Some kids will stop using the potty chair by age 2 or 3 years old if they are potty trained.
You can use a step stool to allow the child to reach the regular toilet and also use a potty ring to put over the toilet seat so that the little child's butt does not fall through the regular toilet seat.
However if the child is not comfortable using the regular toilet then allowing the child to use a potty chair until they are ready is okay.
Even at 7 years old it's okay for them to use a potty chair.
It's certainly okay for your 7 year old nephew to use a potty chair if he wants too.
Nothing wrong with it.
It's just a portable toilet and if he wants to use the potty chair instead of the regular toilet it's okay.
My 5 year old son has a potty chair in his room that he can use if he wants to in the night instead of going to the toilet down the hall.