Football is a very dangerous sport, even with the amount of protective gear that football players have. As you get older it is harder for the body to heal from injuries. This is particularly true if you already have injuries that you carry with you. Coming out the retirement could cripple him.
Whether or not that right situation is in Buffalo is a matter of debate. If it's the best possible quarterback that Harvin would like to play with, he may or may not be able to do better than Tyrod Taylor, who played in the Pro Bowl as an alternate last season. But if it's playing time he's seeking, then the Bills' depth chart (he'd compete for the No. 2 job with Robert Woods again) and a coaching staff with a strong affinity for him may be quite appealing.
It's true that Percy Harvin has endured his fair share of injuries during his time as a professional football player, but that doesn't mean he shouldn't keep at it. The mark of true strength is having the strength to persevere even when things get tough. Harvin is clearly strong both mentally and physically, which makes him a great candidate for coming out of retirement and getting back on the field.
You can't let unfortunate events of your past dictate your future. It's not like he planned to get injured at any time. Percy is only 28 and has some good years left in him. If he feels that hes healthy enough to play and play well than I don't see why you shouldn't let him. Let him prove himself.