Actually other than it being unsanitary, any major concerns such as HIV or any disease, etc would be none as any type of virus doesn't like long once airborne or if it's not in it's natural environment. Brushing teeth and rinsing mouth is about all you can do. Plus most of these diseases do require direct blood to blood contact or skin to skin contact.
So at this point don't stress over this and maybe do a quick look just to make sure this wouldn't occur again. Unless he/she can remember not to do this again.
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