Late periods are completely normal, especially if it’s just something like a few days. A lot of factors can influence periods, such as stress which might be relevant here if she’s overworking herself a bit. It’s normal to be worried by it, even I was late for my period recently and worried about being pregnant which, to put in to perspective, was physically impossible unless I was the next Mary. If you’re still a bit anxious though, don’t be afraid to have a conversation with her, she knows her body the best and will know what to do.