it only messed up the activation if he left a different ROM on that phone.
that phone comes with a special version of android 2.2 that virgin mobile made that has all the features for activation and stuff.
most people root, then upgrade to a different rom that doesnt have those features, becuase they already activated their phone on the stock rom.
your going to have to re root it (which doesnt affect it being activated) and then use the root to install a custom recovery, and flash either one of the 2.2 roms that DO have the activation features (i think bumblebee, in the ROM section of this forum, does that) or restore to the stock android,
everything you need to know is in the rom section of this forum if you read around enough.
something i need to point out right now is that if a phone is rooted. it does not affect anything other than ALLOWING you to flash new ROMs and allowing you to run apps that need "root access"
root alone does NOT break features like activation. so why he unrooted it, i dont know. but youll need to get it back in order to FIX this problem. lets get that clear now so you dont go trying to flash a rom without root. haha