I'd say freeamp2 for leads and clean/semi-dirty stuff:
http://asseca.com/frettedsynth/index.html (with this, I've found SIR with impulses to not be necessary 'cause there's a zillion cabs you can select, from 4-10s, to 2-12s, 4-12s).
for main rhythms/crunch, I'd say use one of these two:
-jcm900 clone: google
-the soldano clone (juicy):
http://www.bteaudio.com/software/Juicy77/Juicy77.html
in combination with SIR and guitarhack's first round of mesa cab impulses (those just happen to be my faves). It's nice to use both the marshall and soldano together - they compliment one another. Follow the directions in the sticky for recording guitars as if your going to "prepare them for reamping".
Also, experiment using a guitar signal that already has some dirt on it from a TS or other OD pedal.
I'd plan on not being able to monitor during the recording; but if he's a good guitarist, he'll be able to play his lines clean and then apply the amps after his tracks are recorded. The price you pay for no amp.
or, like I've said before, just drop 30 bucks on a behringer sansamp clone, apply some SIR w/impulses and it's a done deal.
