Just weeks ago, Kelly underwent 12 hours of surgery to remove oral cancer on March 28. Surgeons also rebuilt his upper jaw at that time.
It's the third time in five years Kelly has been fighting cancer.
Kelly wanted to get a close look at top pick, quarterback Josh Allen. The seventh overall selection out of Wyoming was almost speechless, and left in a state of awe.
"I'm shaking," Allen told reporters. "One of the greatest quarterbacks to play the game and one of the greatest quarterbacks in Buffalo history."
A frail Kelly arrived about 20 minutes into practice, walking with the aid of a cane.
BuffaloBills.com said Allen and Kelly walked off the field together with Allen listening intently to what Kelly was saying.
"It was awesome," Allen told the team's website. "For him to come out and show his support and show everybody that he's doing well speaks volumes. You want to talk about a tough guy, that's a tough guy there to be what he's been through. To come and still be himself.
"He said to embrace the city of Buffalo. He said a few more things that I'll keep to myself. I'm still shaking from meeting him. He's a special man."
Kelly declined to speak with reporters on Friday.