Hey @daniel.kessler - honestly everyone is different. I know many athletes that enjoy training in the evening. But for me, I always aim to do my harder intervals in the morning as that is when I feel the strongest. If she is eating carbs throughout the day and not doing intense physical activity, her glycogen stores should be fine. I would recommend she try to train in the morning and see if she feels like the sessions are more doable.
Also important to note that her cycle could impact her ability to hit hard intervals. During the high hormone phase (around mid week 3 and into week 4 of the menstrual cycle), females typically experience greater fatigue which often increases over the course of the day… along with quite a few other physiological changes that make high intensity efforts tough.
You can read more on that in this thread I posted in a while back 