Re: Is it sanitary to drop a coin into a beverage?

In a word, no. Unless your friend carries sterilized coins that he unwraps 
just prior to dropping them into the glass, the coins are not sterile and 
could conceivably contaminate the beverage. Now, if he is relatively young 
with a strong immune system this may never cause him to become ill, but 
there is no guarantee and the fact that "only he has recently touched the 
coins" is irrelevant.

"Microbiology. 4th Ed. Davis, Dulbecco, Eisen, Ginsberg"