Swimming shouldn't have any effect whatsoever, I spent 4-5 hours in our pool on Sunday (in total) and my enhancements are still as good as they were beforehand. I never see any problems with my client's nails either, they all have pools and use them almost every day.
Make sure they have Solar Oil (or your system's equivalent) to condition their nails/enhancements daily and they shouldn't have any trouble :green: