If his tissue is shredded, he's pretty well toast. You will know he is dead when you smell the awfulest stench you have ever smelled. Put him in a bucket and leave him alone.
The peps can serve in the sump if you plan to get another.
Peps will eat aiptasia and, I believe, majanos, and they sometimes will sample a polyp, then decide to leave it alone---never had them do real damage even in an sps reef.
Unfortunately nems are on their menu. Never heard of them going after one this large, but there must either have been an injury to the nem that got him going downhill and prompted them to act as undertakers (they smell things in the water) or they were just real hungry for nems.
In any case, if his tissue is too wounded to hold pressure, he's pretty well done for.
