Of course he can. Anyone can sue anyone for anything. Did you really think there might be a prohibition against suing?Can he sue for this?
Of course, the ability to sue has little to no correlation with the ability to win.
I couldn't disagree more with the person who assessed your (former) friend's chances of winning as "excellent." Let's say the guy does sue (and let's assume that he's either represented by a non-moronic attorney or has the ability to draft a complaint that can survive a pleading motion). There's virtually no way that a case like this would be decided by a motion for summary judgment, so the only way for the plaintiff to "win" would be by taking the case to trial. Only 10-15% of civil lawsuits go to trial, and I submit that taking a case like this to trial would not be well advised. Let's be charitable and say that there's a 15% chance of the case going to trial. Based on the limited facts given, it's probably a toss-up who would win (it seems to be a "he said; he/she said" case), so that means there's maybe, at best a 7.5% percent chance of the plaintiff "winning." That's hardly an "excellent" chance.