From you description it seems that the nic level and PV are fine for him when he drips, but not so satisfying when he uses carts. It sounds like his atty isn't getting enough juice when he's using carts.
To me this is the real downside to 510s, they are great, as long as you are not starving them. You have several choices to try and address this since dripping doesn't sound feasible while he is working.
Look at some of the cart mods and see if that helps. Each has their pros and cons and usually comes down to ease of filling vs performance. The ZFM (zero filler mod) works great, it uses all of the liquid in the cart and last much longer than mods that require filler. However, they are not easy to get right, it may work well on one atty and not so well on another. This is due to inconsistency from atty to atty (bridge height, alignment, etc). Also refilling them isn't as convenient as a filler type mod. Blue foam, polyfill, and other fillers work great but tend to loose their wicking ability after some use. They are easier to fill and top off, but will not last as long as the zfm.
Another choice would be to look into cartomizers. They are now available for the 510, or you can use a 901/808 adapter and use KR808 cartomizers. You can get them prefilled or emplty and fill them with your own juice. And since they are the atomizer and filler in one piece, you are able to switch flavors without any residue or residual flavor like regular 510 atties.
I use 510 cartos while on my commute or when dripping isn't doable, but I primarily drip when possible. The cartomizers do last much longer than carts, the 510 cartos can hold 20-30 drops of liquid.