That's an easy one. Palms forward in a (heavy) dead means
(a) he might be trying to curl the bar which would give him biceps pain (might not see any movement, but the activation might be there). A 50 lb. unilateral curl would be awesome. How much was on the bar again?
(b) he might not have the shoulders properly retracted, in which case the biceps is trying to stabilize his shoulder joint during the lift. It's harder to keep shoulders in position with a supinated (palms forward) grip This could cause radiating pain down the arm exactly as described. Had it before myself from bad overhead lifting form a few years ago.
The biceps can't stabilize the shoulder well and it certainly can't do it and assist with the lift. Oh and yes he is on the verge of pulling or straining something which will probably end his deadlifting, possibly forever (if he blows out a shoulder, that is).
I never DL with alternating palms. Had a biceps tear once and golly do they suck. Get him to do some heavy-ish rows (chest-supported, palms facing to his rear) where he really pulls the shoulder blades back and holds for a count of 3 at the top to activate everything. Then try again. Have him switch to palms facing back, really lock down those shoulders before starting & lock out those triceps before the lift.
Course, I'm not a trainer nor a medical doctor nor do I play one on teh Interwebz... YMMV