I don't see why he couldn't since it's just software based. Most of the 3.1.3 update was for itunes and safari. He should be able to google 3.1.2 firmware, find the actual file and download it. Before he loads that he'll need to make sure and back up all his files he wants to keep. From there it's as simple as plugging in the iphone, going to the restore page, holding down SHIFT while clicking restore, which should bring up a window where he can load the firmware. Load it up and then I'd go and find Blackrain and use that to jailbreak it.
I downgraded my ipod touch just last night to 3.1.2 just so I could jailbreak it.
It's a much different story with the iphone 3gs.
It'll heavily depend on two things:
1. His baseband
2. If he backed up his iphone information with cydia or another server
If he has the older 3gs baseband, he can download a program (forgot the name right now), that will falsely sign the firmware and load it to his phone, jailbroken.
If it's the newer baseband, there isn't software available to do that yet, but if he backed up his phone's info via a cydia server or another means, he can have itunes falsely point to it for firmware verification, and downgrade the firmware.
If it's a newer baseband, and he didn't back up, or it's a brand new phone with 3.1.3 already, he's fucked for now.