She could endorse him, not actively campaign for him. That would pay back enough for any supposed debt she may owe him for bringing her into the national spotlight. His silence in not defending her was deafening – in light of that, he’s luck to get anything from her. Among her supporters (conservatives) this will hurt her.
I agree with the loyalty issue, and I agree with your proposal. Sarah could have made a vague one line statement of "support" and be done with it.
Anything more is supporting a guy that wants to bed with democrats on the healthcare bill that McCain wants to negotiate with Pelosi and Reid. Then there's McCain's stance on amnesty, which is illegal.