Does he have "after market-clamp on" battery terminals? If so that is most likely probelm! I am guessing that you mean by "not starting" that it does not "crank"! and it is not (cranking) starting because of corroded battery terminals!
You can remove connectors off battery posts, and soak them in glass of water with 1 tsp or so of baking soda mixed in, -- all corrosion will dissolve-- (noted by "fizzing"), -- When little bubbles cease to rise,- the terminal is clean! Now contact area can be covered with vasolene, or light grease to keep air out and oxidation will occour more slowly! Flush all cables, and battery and carrier with water, - this should remove all acid from area.. Now you will be able to tell later if battery is "oozing" acid! Washing down before flushing with soda water mixture will remove ALL acid from area, -- if you go and pour a little on battery in a few days , - and it fizzes again, then you have acid leaking out somewhere! (Be carefl not or force any down inside around filler caps (if it has them) as it will work to neutralize acid in cells also!
Now about those "clamp on terminals", get rid of them (cables and all) and put on new cables with the ends attached right, -- you increase places for corrosion 100% by putting on those "cable ends", and now you have 2 places for corrosion on each cable end, instead of 1!