Last time you'll see him will be just by the end of chapter 5-2, he'll let you know that's "as far as he goes" so you'll know you won't see him again, after this, there's gonna be about three more red gems available.
If you're going for the High in Las Vegas achievment/trophy, here are several tips:
- Use a collectible guide, I used this one, but it has a couple of mistakes, namely the 2 red gems mentioned in the chandelier section and the second one on 5-2 which I never managed to find on all three playthorughs.
- Play on Lemon Hunter (Easy), red gems cost 45 white gems (Vs 75 and 120 on Medium and Hard respectively)
- To maximize white gems, shoot enemies in the legs then stomp them. I recommend investing early in the Boner's damage so one shot takes off the leg completely.
- Some red gems can be farmed by a glitch. If a red gem is collected before a save point, and the red gem's location is still accessible after the save (i.e. the save point isn't behind a closed gate, ledge, etc), you can quit the game, load the save and go back to where the red gem was to collect them again. During chapter 4-5, there are two gems that can be farmed a couple of time this way.
- There's another glitched gem early in the game (the second gem available) which be collected indefinitely, the method is time consuming though and may take a few hours to collect all 80 gems, but if you messed up the trophy and don't want to play the game start to finish again, it might be a viable option. see the following video for this method