does a softmod only change shit on the hdd or does it effect the mobo. Like if my friend does a softmod and screws up can he put in a new hdd and a modchip?

It only alters files on the hdd (additional font files, ect)

You friend will be ok to do what he's doing.
Also, do all the usual things you would with a softmod, eeprom backup, aswell as C:/E: backup.

would it be possible to buy an unlocked hdd that is softmodded then lock it on his xbox

No you can't. You need to use xboxhdm to lock the hdd you're buying.

Possible to lock it but not in his xbox, he would have to use xboxhdm ( the xbox would not boot up with an unlocked hdd attached). if you have his eeprom and the drive is lockable, but i dont know of anywere that sells softmodded hdd's.