LE wasn't born yesterday. I've seen posts that say Return to Sender doesn't work anymore. Also a signature is not required.
What is required is that you take possession, no signature required.
And no matter what that doctor says, in the eyes of the law a valid medical prescription is required. Valid in this case means that the item being prescribed can only be imported if the medical treatment was started in the country where you are receiving the item from. Valid also means there had to be a physical examination involved.
So if your friend didn't go to the country to have this treatment started and/or never has seen this doctor in person, the prescription in the eyes of LE doesn't exist.
Also either you get a seizure letter from Customs or a controlled delivery. Not both
If you want more info on the legalities, do a google search for "Rick Collins"