Since it was built as a kit he is very familiar with it. Open it up and very closely examine all parts for heat related stress, maybe a bad tube was drawing to much current. Also check every solder connection, even reflow them, especially if this was his first kit build. Unless he is experienced with soldering, this is a common problem with first time kit builders.
Report back with what you find.
BillWojo
Report back with what you find.
BillWojo