Blaxican is correct.
The person would have to be found, within reason, legally mentally competent to make the decision to commit suicide. Also, only a licensed professional should perform the assisted suicide, not family, friends, etc. (Signature and a witness.)
If the person is not able to legally make a decision for themselves, then a family member based on the states specification of guardianship (such as Texas, where the oldest daughter makes that decision, etc.) will have to give consent.
If the person did not have any relatives to give consent and the person is unable to competently give legal consent, then a state officiator will operate on their behalf. This officiator will be the source of controversy as some may be too liberal and some may be too conservative.