Chris

As Paul says check the voltage engine running. A little over 14 is fine a little under with lights etc turned on is probably normal. The amperage capacity (physical size) of the battery would have nothing to do with fuses blowing or not, it still puts out and operates everything at 12V. To trace the problem requires hands on troubleshooting and knowledge of which fuse(s) are blowing and under what circumstances. Can you assume he is installing the correct size fuses and not something smaller? Are they blowing as soon as inserted or when an accessory device is turned? Is this just random intermittent blowing of different fuses? If it is, that sound a bit scary.

You probably know this procedure already but if a fuse blows immediatly upon insertion or turning something on then connect a small bulb across the fuse connectors, it will light, then go to the wiring and accessories associated with that fuse and search for the problem.

I would not recommend an "intervention" here, the risk is high, imagine if wiring caught fire (yikes) after you inspected it. I think it best to stay in the background even though you may find that difficult to do.

Good Luck