I've wondered a similar thing to the OP on occasion, though not with crafter's fortune.
My wizard isn't very strong, so likes the idea of getting a pony/horse/riding dog/etc to carry all his gear [insert obligatory reference to buying a handy haversack when funds allow].
Can I buy an untrained horse/etc and ask the druid I just went adventuring with to train my horse for me in down time?
My gut feeling as a player and a GM is the answer is no, because it boarders on the transfer of wealth between players, not enough to be significant, say 50gp for buying an untrained horse and maybe 20gp for helping someone with a dayjob check, but since wealth transfer except under very very limited circumstances is a no no, then my assumption is it wouldn't be allowed.
For thematic reasons, what if the wizard/alchemist/whoever is providing crafter's fortune wanted to go adventuring a couple of weeks before the person who they were providing the spell for.
Crafter's fortune assumes you are casting it each day in your downtime, since downtime is undefined, I don't think that you could necessarily guarantee that the spell had been cast each day. Take for example both people did The Glass River rescue together, the fighter takes a break while the wizard does Destiny of the Sands and they both adventure together again in The Merchant's Wake. I know downtime is undefined, but I don't think there is any way to say that the period the wizard spent doing Destiny of the Sands didn't happen for the fighter.
...I feel like I'm about to get into the problems with time travel so I will stop there :P