If they were a true gift (as opposed to a favor or a kindness), then you should feel free to do whatever you want with your tickets. A friend would also understand about taking care of a sick relative. Personally though I think you should seize the chance to go with your fan friend.
True story:
If it's been a good year, my employer will raffle off several thousand dollars in prizes to the employees just before Christmas. They're a very generous business. It was a long shot but I was definitely fantasizing about winning one of the iPads or gaming consoles.
One of the prizes was tickets and parking to an NBA game in the New Year. I am not a sport person. I remember literally having the thought "Oh man, that's probably the last prize I would want in this group" when my name was called. I took it as a sign about being grateful and decided to go.
Over Christmas however, I caught a cold from my toddler niece and was sick as a dog for days. I arranged for DH to drop off the tickets at my workplace so a coworker could use the tickets.
A week or two later my boss (the Principal who donated the tickets) stops by my desk. He had heard I had been sick and couldn't go to the game. I tried to assure him that these things happen and I didn't mind, but he insisted on giving me a new set of tickets for later that month. We went and cheered our home team to victory. It wound up being a lot of fun and I'm glad we went.
In retrospect, I think I got the best prize in the lot. I could easily purchase any of the prizes in the lot if I really wanted to, but buying lower-level, mid-court seats at a well matched game would've been a lot harder. What I really got was an experience instead of a thing and I'm grateful for that.