He's going to see it sooner or later, and it will be his, and the paperwork he fills out to buy it from the FFL & the FFL's logbook will clearly show that it legally was no longer yorus and no longer your issue, so I don't see the harm, especially if he's paying you for the gun. He may *need* the serial number to have the gun added to his permit before he can take posession, if he lives in one of *those* states.
As far as I know, the reason for covering serial numbers in pictures is twofold, both related to worry about bad repercussions:
1) worry that someone will take your serial number and file a stolen firearm report, acusing you no end of problems if your purchase was not from a FFL who'd have record of selling you the gun. If you've previously bough a gun face to face and don't have "proof" of ownership, that'd be a very valid concern. I'm guessing a 5.7, as a fairly new model, is probably clearly documented as being yours newly purchased from a FFL so no worry there
2) worry that someone somewhere is making a database of firearms that everybody owns. That's a bit out there, and you're going to have proof that you sold this one via FFL transfer, so when the paranoia goons come to get your 5.7, you have prrof that you're not hiding it, it went to a FFL.
I'd send him the serial number, no problem.