First I hope he/ she knows you are calling them out as poor Dps . Seems kind if underhanded if this is a secret from them.
First , 2 if these fights are hard to judge properly. What is his role on Tortos? Does he kill turtles the whole time? Then he's probably doing ok, it's not a strong fight for us unless we are chain lightning the bats.
Horidon have him cl more on adds.
For Megaera have him always keep fs up on the second head. It greatly increases lava surge procs.
In terms of talents have it drop elemental blast and grab pe. The rotation is easier and Dps is equall if not stronger. Then have him pay attention to his 6-7 stack earth shocks. They add up over a long fight.
In terms if gear drop the lei shen trinket and get wushy one off council. Much much better for ele ( don't let him vendor if though we may need in next patch)
Just had a quick look at the logs and his armory. Honestly for 519 ilvl, no meta gem, no 4p, he's about where I figure he should be provided hes not doing anything to cheese the meters. Maybe have him try out PE just to simplify the rotation and I find it is superior on a lot of fights in ToT. He could also replace the lei shen trinket with the vp one. Really his damage will depend on the strats that you use.