Welcome to the forums.
Unless your mate is absolutely in love with the case, why not replace that too? It hardly seems fitting (to me) that they would replace the guts then keep a 10 year old tower, unless that is something they plan to do sooner rather than later.
One way to tell what motherboard is compatible with the tower is to count the expansion slots on the current motherboard. If it only has 3, and no room in the case below thge board, its a mATX case. If the current motherboard has 5 expansion slots its ATX. most cases are ATX (mATX is smaller so will fit by default.)
If the case is 10 years old it might also have a 10 year old power supply, so I would get a new one of those too. Theres no sense in buying nice new fancy guts for the case then frying it a week later due to an old crappy cheap worn-out gateway PSU.
