I think it is difficult to answer your question without knowing what her employment contract says.
It's possible that a boss asks her to do supplementary work, but the first think to look at is the contract.
Moreover, supplementary hours aren't always "paid" in monney. In some companies, employers don't get paid but can deduce the hours they worked in supplement and get free days and even week. Actually, nowadays this practice is very common in the swiss economic system, one needs to be flexible.
If your friend has some doubts about her contract, I'd advise to talk first to her boss and then if necessary go to HR in her company to clarify things.
The first precaution though is that she carefully registers when, at what time, how long and for what kind of work she had to work supplementary hours.
Good luck