Yep, if he is maintaining then she can continue doing what she is doing. However, "good teeth for 30" can be no teeth in very short order. My old man lost his first cheek tooth 2 weeks after having them checked and my boss going on about how good his teeth were. This is because somewhere between 28 and 30 years of age a horse starts out-living his teeth. They are born with all the root their teeth will ever have in their jawbones and over their lifetime the tooth continues to grow up into the mouth to compensate for normal wear from grinding. So eventually there is just so little root left that the tooth can't grow up as far as it should and that the root isn't enough to hold the tooth in place.
If she did change him over to a senior feed that is balanced better for his age (seniors have a little different nutrient need) and more easily digestible, she might be amazed at exactly how good a shape he could be in and how much energy he would have. My 33 year old routinely goes running around the pasture with his tail in the air--sassy old man!
Licensed Veterinary Technician