Check in around on the disease forum for treatments and photos of sick fish. It sounds like either Brooklynella or Ick. If it's the former, he has very little time and you will need to treat with chemicals, probably Formalin (if memory serves). If it's ick, you have a little more time. You'll probably want to put both clowns in quarantine and treat with hyposalinity. It takes a few weeks and you must do it very carefully, but it's the best option. You can find complete instructions on the disease forum.
Do you have a quarantine tank you can move him (or them) into? If not, you should buy a cheap 10 gallon tank, a simple HOB filter, a heater, and a small powerhead and set one up. You may not be in time to save this clown, but is friend may well take ill before long and you will want it.
Good luck!