Informed consent is the process of telling the patient what will happen during surgery. The doctor or assistant also goes over any possible risks and problems that can happen. Everything should be explained at the patient's level in easy terms that can be understood easily.
The patient can ask any questions he or she may have at that time. That's the "informed" part of informed consent.
The consent is the patient's signature on a form stating that he or she understands the information and agrees to the surgery. The patient's signature shows that it is a voluntary act on the part of the patient.
Offer to stay with your mother-in-law during the appointment. Make sure she fully understands what she's signing.