One sip.
A friend of mine accidentally took a drink of a glass of water a man brought to the table- it was intended for the females but they had left. He thought it was water but was a gin drink. He stopped at one sip and went to his hotel room, where he violently vomited and then woke up fours later on the bathroom floor. He swears the man drugged the drinks intended for the women.
Maybe your friend should take up wearing nail polish.