Friar is usually found patrolling the corridors of the Bandit base and the outside compound. The Contacts list in your diary will tell you which NPCs are nearby, but it has a limited range. Check it in a few locations around the base.
Friar can get killed during the military attack, in which case you should find his body somewhere -but you will have to search for it. You should be able to recognise him, as he is one of only two Bandits in the base who wears an exoskeleton outfit.
If Friar is dead and you leave the Dark Valley without finding his body, it might disappear by the time you return. In that case you may have to spawn in his PDA.