I downgraded my 3DS and never lost my mii plaza data (which means a lot to me). Just make sure that when he downgrades the FIRST thing he does is make a NAND backup. Then, if something goes terribly wrong, he can restore that (with a hardmod).
Streetpass is weird because it saves to the NAND, so its save data will be encrypted per-device. It is only possible to re-inject it if you have extracted movable.sed (a NAND file), iirc. (See: "manual system transfer" somewhere in this forum)
If he is worried about the part where you format your SysNAND to unlink his NANDs, re assure him that it will only affect his sysNAND and not his emuNAND. Make sure he follows the tutorials though, it would be bad if he forgot to create an emuNAND and accidentally deleted his stuff...
Of course, while it didn't happen to me and many others, there is a very, very small chance of a brick witch will (obviously) destroy all of his savedata. As long as he/you are careful, it should be fine.