Dear Ada -
The main site of the Mayo Clinic is located at 200 1st Street, S.W. Rochester, Minn. 55905. Their primary website is
www.mayoclinic.org although for some reason, I'm having trouble getting connected right now.
There are also major centers in Scottsdale AZ and Jacksonville Fla., along with a host of smaller feeder clinics in the upper Midwest and a facility in one of the Persian Gulf States, Dubia if memory serves.
If your friend is planning on traveling anyway, s/he might as well go to to Rochester, but probably on or with the referral of a treating physician. (It's not required but it's certainly advisable.)
Mike