How many classes a day? And what style? Certain styles of yoga might put a bit more strain on the musculoskeletal system, but generally speaking doing an hour of such exercise daily should not lead to overtraining.
He/she is also not doing a single repetitive activity, but varying the stresses with different disciplines. And each of these activities has similar, but unique movement patterns and benefits.
The only thing I would say is that it is important understand that although these practices are not (usually, though I’ve seen yoga done with impact) low impact, they are not going to be the same as a low impact cardio workout. Tai chi, perhaps… I am less familiar with that. But even power yoga is not going to give the same benefits as walking, or biking etc. So it might be helpful to them as they are getting the flexibility, balance, core training, etc, they might want to consider adding some cardio work, if is possible for them.
And it is fine to have a regular day off, to recharge if they start to feel overtrained. No schedule has to be set in stone forever.