well, it is sort of like filling up your pot of boiling water with ice as fast as it starts boiling, LOL.
But, antibiotics usually win out and the probiotics are generally meant for the GI tract specifically.
Are his antibiotics topical? If so, then I would think it would work fine. If they are oral, then take them a few hours apart so the antibiotics have a chance to absorb through the GI tract.
He could also try active yogurt instead of the probiotics, for many people that works well, too.