If the garage roof is framed over the roof of the house (where the roofline looks like there ISN'T a garage), then no, it's not an exterior wall. (That's one perspective because there are PLENTY of homes that have the garage designed right 'into' the house. In fact, some homeowners put curtains in the garage windows to make it look like a 'room'!) If, however, there's a clear 'distinction' (or line of demarkation (sp) between the 2 'structures', yet they're 'attached'), then it COULD be considered an exterior wall. Obviously, if the structures were separate, it's an exterior wall.
Proper landscaping, properly hung gutters and downspouts, and appropriate drainage should add to the 'prevention' of water in the basement. DO take a look at this during your next rainstorm. (Watch out for lightening, and look UP to the roof of the house as well as DOWN.) You may find leaks, over-flowing gutters, MISSING gutters, clogged downspouts, water running back TOWARDS the foundation, and who knows what else. If you suspect that water might be coming in ANY wall, then have it 'addressed'.
Now, as an aside, if you literally have water seeping through the walls, the Waterproof Coating that's applied will eventually fail. Condensation is one thing but seeping is another. Address the gutter, downspout, and landscape 'issues' I mentioned earlier. Consider a perimeter drain and sump pump. Waterproof coatings on the INSIDE of the house are only bandaids.
Now to boot, IF YOU'RE TALKING ABOUT EXTERIOR WATERPROOFING, just do all exterior walls. Period! If you or he wants to do more, then just work it out. I was originally assuming you're talking about INTERIOR waterproofing.