From what I quickly found on teh Googlez -
An eTrigger for a T98C runs about $100.
A Response trigger for the same marker is roughly $70
The marker, new, was about $120.
Your buddy is looking at putting almost 100% of the purchase price of his marker, back into the same marker?
No matter how polished and glossy and nice you believe you can get a T98 to be, it will always be a T98. It will perform its function, but will not be anything special. Pumping money into marker upgrades for the sake of upgrading a marker is probably the single worst use of finances in paintball, especially at entry level. For $100 more, your buddy can look into a used FEP Quest, Invert Mini, one or more of the Smart Parts markers (though I'm not openly suggesting them) - all of which will shoot, function, and perform significantly better than a T98C
A Spyder Fenix/Azodin Blitz is roughly $50 more than the eTrigger, and includes firing modes, electronics, an electronic trigger, eyes...
With the T98c, you get... a run of the mill rental marker.
The only suggested upgrade for a T98 would be a J&J ceramic barrel. Beyond that, push your money into buying paint, or saving for your next marker.
You can only polish a turd so much...