My chickens get fruit in season. You need to use caution with any treats, as has been mentioned. If their production goes down, pull back on the treats so their diet is more balanced. It sure as heck isn't going to stop them laying for a month! I figure in the heat of the summer, I'd rather them have some watermelon and enjoy it than get that extra egg, if indeed it affects their laying. I've had some mild reduction in laying that correlates to the time I've been loading up on the fruit treats, but as for the variables: it has also been freakin' 100+ degrees for over a month, which coincides with the reduction. When the water in the waterer gets a little stale, the production goes down, too.
Short answer: fruit is OK. Everything in moderation.