Rolf, all plastics outgas (that "new sail smell") to some extent. They also oxidize (that's what makes them brittle). Heat accelerates that process.

Granted, a mylar-based sail will hold up longer to that treatment than a dacron based one, but heat is not good for a sail. He probably wouldn't notice the difference over one season, but heat will definitely shorten the life of the sail by making it brittle before its time.

I've seen sails (dacron admittedly) that have been left in sailboxes unused for several years. They shattered when we tried to unroll them.