Could they remove the pectoral musle in a voluntary mastectomy?

Could : But why? Radical mastectomy for cancer historically removed pectoralis major as part of the dissection. This adds some risk and some cosmetic issues.

Get 2nd opinion: This procedure of removing the muscle is not considered standard. If there is muscle involvement, then you need chemotherapy before surgery. I recommend you ask more questions and consider finding a breast center to speak with a breast cancer surgeon for options. Good luck!

Depends : It depends on how locally aggressive the cancer is. Most of the time, muscle is not removed.