My understanding is that a major issue for anyone playing an instrument following a Mastectomy and Rebuild, is that typically, the tissue and muscle that is used in the rebuild comes from the back/shoulder region and this can cause problems of strength/control/pain afterwards for a musician depending on what instrument they play. A close friend, (a Scottish Small Piper) faced exactly this dilemma and through close discussion with her surgeon (another piper incidentally) the choice was made to take the rebuild tissue from her stomach area rather than from her back. Once the scars had healed she was able to play again in the normal way, (they are elbow pipes), though the opperation itself took slightly longer to recover from because the incissions are in a more "mobile" part of the anatomy and therfore more subject to delay in healing because of disturbance through normal movement. Maybe this might be a possibility for your friend to consider?
The operation was unusual aparently, especially as my friend was quite pettite, and the medical team took many photographs of each stage in the process for the purposes of academic resurch which should now be available to other medical professionals. PM me if you want further details as there were some consequences which your friend should be aware of in how best to plan the surgery and any subsequent treatment.
best wishes to your friend.