Can a company withhold PTO from an employee who’s quitting?

My friend is quitting and she has over 100 hours of pto saved up. Shes leaving on the 22nd. The company told her “since you gave us 3 weeks and not 4, we can’t give you your PTO”. Understandably she’s pissed. Are they allowed to do this? I told her why not threaten to sue? Even if you don’t plan on going through, sometimes the threat might work. But we both don’t actually know if what the company is doing is illegal. Can they do this?