I wouldn't recommend asking her to carry a rider just yet. The added weight would just increase the awareness of the pain in her foot. But, exercise is good, so keep up with the turn out, hand walk her or even lunge her if she doesn't seem bothered by that. Exercise will keep good circulation going in the foot to help it heal faster.
Depending on where the nail went in, it could take longer to heal. Soft tissues, like the heel bulbs tend to close back on itself and not drain as well, slowing the heeling process. The other down side is in the front of the foot, she could have even nicked her coffin bone, and which (in worse case scenario) get infected and become eroded and permanant lameness could occur,or worse. WHen the nail puncture is closer to the perimeter of the foot (where a shoe would go) the better the prognosis. You said it was in the frog? Just give it more time. Perhaps months, as the infection may not be all gone just yet and still have some abscessing to do. Just be sure to pay careful attention to what's going on with her comfort and check her foot every day and keep in touch with your farrier. His/her hands-on examination will be more conclusive than my online guessing and spouting of "worst case scenarios"