Shower single handle hot water won't shut off - replaced catridge

[Dudz]

My older parents have a Gerber single handle shower unit with tub faucet/diverter. When off (& in any temperature position) - hot water runs/won't shut off.
We sourced & replaced the Kerox cartridge but hot water still runs from tub faucet. Is there any other option or does this require professional valve replacement (& shower wall redo)? They worry about bringing in a plumber during COVID-19. Attempting to share photo of unit seat

Thanks in advance.

 

[Dj2]

If you pull the cartridge out, you might find a damaged or misplaced O ring, and chances are that this is the problem.