they say that there are a lot of contributing factors to MHLLE, from nutrition to stress. most have been mentioned here, but i would suggest reading this Steven Pro article ... i had a Chevron Tang and Sohol Tang that had an, almost as bad as your case, outbreak of MHLLE. now, though, you can barely even tell they ever had it. mine was due to the 2 fish being kept in a tank that was to small for even one of them. i do not have any pics of before but here he is after a long healing time and being moved from a friends small tank to 1 of my bigger tanks ...
you can still barely see the lateral line scar on his side, but when i got him ... he had very little fin left. the Sohol made an excellent recovery and has since been sold.
here is the article that i refered to earlier, it really helped me to understand a lot of the factors that are believed to contribute to MHLLE ...
Marine Head & Lateral Line Erosion: A Description of the Syndrome and a Review of its Speculated Causes by Steven Pro - Reefkeeping.com