**updated with links and rounded up prices**
$40 - amazon carbon lcg rails
$5 - g speed titanium spacers x4
$80 - 2 speed with rear freewheel and dig
$15 - ally gear cover **last thing i added before it wouldn't stand it on it's side**
$15 - plastic gear cover
$35 - steer servo
$15 - amazon chassis servo mount
$65 - aux servos
$30 - brazin pro skid
$20 - stainless links 1 says 324 wb, but as mine sits its at approx 313
$20 - stainless links 2 says 324 wb, but as mine sits its at approx 313
$75 - previa shell
$80 - transmitter
$90 - fusion se 1800
$30 - 3s lipo x2
$40 - injora te37x copies
$50 - crawler innovations dual stage - sourced set locally
$45 - jconcepts ruptures - sourced set locally
$25 - desert lizard 100mm front - sourced pair locally
$20 - desert lizard 90mm rear - sourced pair locally
$80 - traxxas stock trx4 portals with working lockers - sourced pair locally
$free - designed and printed sliders, front + rear combo trays, front + rear bumpers/rail end caps
$free - front and rear driveshafts from my cross JT4
It is definitely a mountain of information to take in. I built a fairly cheap quad to practice flying and it does take some commitment, but once you (he) figures it out it will be smooth sailing. I think buying him parts is the way to go as well.
Here's some of what I used. There is almost certainly better stuff, but this should work.
PDB--$10
Flight Controller--$27
Controller and RX--$50
Battery--$17 (buy 2 or 3, they only last a few minutes)
Foam Landing Pads--$7 (Not needed but make for slightly softer landings)
I think that's all you need to get started. Not sure if he has soldering equipment, but that will definitely be needed. Again, can't say definitively this is the best or cheapest stuff, but I think it's everything.
EDIT: I left out anything specifically for FPV. If he hasn't flown much or at all it will be a while before he's going to need that stuff anyway.