The PV is in the main 125A panel. The subpanel was installed 6 years ago and is a 60A feeding just the remodeled kitchen and a bathroom. It connects back with a single FAT feeder wire across about 60 feet through the attic (one story house) to the MSP.Likely the existing PV is tied into the garage subpanel. I don't see any other way this is connected but I could be wrong.
There is no reason why the existing PV couldn't be integrated, but it needs Tesla PV CT's at a minimum. Also, there will need to be a way to rapid shutdown when complete.
The main panel will be maxed per the 100% rule if he lands a 125A breaker on it. No other breakers can be in that MSP if this approach is taken.