They would have to be able to come up with some evidence, yes. It would not necessarily have to be call outs on Fridays/Mondays although that is the classic giveaway. If they are asking for doctor's notes for each and every FMLA absence that is a definite violation; once FMLA is approved they cannot ask for re-verification more often than 30 days, and then only if there is a reason to believe that the situation has changed.
Suppose the original certification was for three-four days a month, and the employee is calling out five-eight days a month. That's a valid reason to suspect abuse and request re-certification. And if the doctor fails to provide amended certification for five-eight days a month, that's evidence.
Your employers are asking for too much, too often. But until YOU are denied FMLA, you don't have a violation.