Yes he should definitely go see a physician....
it could be something as simple as a kidney stone passing.
My oldest son (15.5) came to mr sageofages and me the other night and said "I think something is wrong!" He pulled down his jeans and the front of his bvd's were blood red! We are talking if he had been our daughter I would have thought it was a period starting!
Needless to say we were at the dr's the next morning. Apparently, no uti, no injury, etc. Dr says possibly a kidney stone (as he did have some pain in his back pretty bad about two days before) was passing and it can be that dramatic.
UTI, Kidney stone, burst blood vessel because of exersion
(sex can be strenous if done properly
could be any one of the possibilities.
Good luck to your friend.