I don’t think what the employee did is so wrong. If the employer did not like it he could tell her not to do it again. Nowadays some people try to signal virtue by being ruthless toward anyone who criticizes or inconveniences a member of a special group — blacks (for example the current socks in pool incident), and in this case a vagrant. Some compassion should be shown to people who do something thoughtless or politically incorrect.
A customer purchased the drink (and cup) for the homeless person. The employee had. no right to deface the cup.
It was stupid and unprofessional. I get we here like the “FU, and get lost, you bum” message, but this was the most idiotic way of doing it.
Can these places stop writing crap on cups and receipts and just dispense the damn product?
If this person is being a pest, call the cops, or let management know, and let them deal with the fallout.