Dave's right. AE is not the right tool for this. You'd need to hire someone to digitally re-create her head, match the movements, lighting, and texture, and then re-composite into the scene. That requires a true 3D modelling application. I suppose you could composite the results in AE, but that's the very last step. Re-shooting with a bald cap is the absolute cheapest, fastest solution.
You can do a morph (RE:Flex from RE: Vision Effects) — from her normal self into another version of her with a bald cap as suggested. It'll be quite easy if there isn't a lot of perspective change. If there is going to be a fair bit of perspective change then you'll need to time the shoot very well PLUS do some re-timing in post, prior to morphing.