Working in a supermarket and offending a customer by being lewd would (IMO) definitely tick the box of 'bringing the company name into disrepute' which is usually a fairly standard clause to have in one's list of examples of gross misconduct as per the employment contract (you should check this).
As regards the fact that his supervisor was aware of it and didn't bring it up with him, I don't think that would help your friend - you wouldn't expect to have to tell staff not to be lewd. I imagine that all that would come about from raising this previous comment would be the possibility that the supervisor might be disciplined himself for failing to deal appropriately with the issue when it was first noticed. It probably won't work as a defence for your friend (although rather bizarrely, had he known at the time that the matter had been raised with the supervisor with no action being taken subsequently by the supervisor, he might have been able to argue that he was operating totally within a framework that had been effectively 'endorsed' by his boss. Still shaky ground, but might give him 'wriggling' room).
I would imagine however that your friend will simply receive a verbal warning or a written censure and that will be the end of it (although I imagine he will be quite shaken - and deservedly so).