10-20 Years ago, I would have agreed with your friend. The balata covers they used to use on high-end golf balls would break down in water (it takes more than a day, though). Balata is the extract of the balata tree, kind of like rubber, and like most natural substances, it is somewhat water soluable. You could tell when they weren't any good because the cover of the ball got dull and felt kind of crackly. Nowadays, Pro-V1s are covered in urethane which shouldn't break down in water.

POSSIBLY BOGUS THEORY FOLLOWS:
Urethane does, however, react to UV light and since sunlight is still strong in less than 3 feet of water or if a ball sits in the weeds for a while, you might end up with "sunburned" Pro-V1s (must resist the urge to call them "sunburned balls" ... ouch).

I'd think that if the ball isn't yellowed that it's still OK. Youve got to be carefull with those used Ebay balls though because sometimes they repaint to make them look new.