Don't know what his chances are. Seems to me if he made it this far he might be OK.
I've had fish get caught up in my ritteri before though, and although not initially killed, did eventually perish.
Happened at least twice - once a pajama cardinal, once a doliatus rabbit (yeah, isn't that great, I wait >2 years to buy one and then he lives for 2 days in my tank because the anemone killed him. Needless to say that anemone was out of the tank at that point, 7 year ritteri or not that was the last straw
). These were smaller fish though and quite clearly covered in burns.
Anemones can be sometimes more trouble than they are worth.