He is almost maxed on the LC-100 so now would be a good time to start using it, and not before.
If he's in a hurry and wants to take full advantage of SIB periods, then I guess any Isis LLC (cheaper, less efficient) or ArMatrix LR-XX (more expensive, more efficient) he wants to use would be ok. If he goes the ArMatrix route, he should use an ArMatrix Extender P20 (L) since then the average markup goes down and guns last longer.
In the long run diversifying a bit will open doors, so maybe start using pistols too. If he is a big grinder an Adjusted EP-41 is hard to beat in cost/performance, so it could be his long term goal.
Good luck.