could be infection setting in....
my buddies wife shot a very nice 10pt 2 years ago i think it was...opening day with a 243...shot was just a HAIR low...with a bigger gun, it would been a dead deer IMO...looked like the bullet hit the top of the leg and couldnt make it through the bone...
2 weeks later, the LAST day of the season, he took our crew there to do some pushes...he spied him bedded on the edge of a SGL food plot and it started to sneak away..away from any drivers or standers, so he put him down. he was really trying to get one of us to kill him (just to be a nice guy, get us hooked on BIG WOODS hunting, and so he wasnt in the DOG HOUSE with the wife

)
he dropped it...infection set in...it was N A S T Y...buck was VERY weak and wasnt going far even without being killed...it looked like it didnt move or eat in the 13 days since it was shot...very skinny and sickly looking...was atleast a 3.5yo buck..you could just look at it and see it was sick...i have no doubt it would been dead within a week or so tops...no way it was getting down the mountain to water...had food right there but didnt look like he was doing well at all...
maybe thats what your buddies buck is going through...
maybe he can find him again and put him down...sounds like he is in pretty bad shape...