I have a friend who has a friend who has a 93 sentra se-r. he has a SR20DET motor inside his engine... and i was thinking... how was that possible? I mean, aren't SR20DET motors for RWD cars not FWD? so can I put an SR20DET engine inside my 02-03 Sentra Spec-v?

In a nutshell, yes and yes

All the cars with the sr20det stock are either rwd or awd, some of the awd versions are setup like a fwd engine, so those are the motors you do the swap with (bluebird, gti-r, etc). Yes you could swap one of these dets into your spec-v, but you would have to get new mounts and transmission.