What do adventurers want? Money.
What's a good way to get money? Sell out your friends to the enemy.
What's a better way to get money? Pretend to sell out your friends to the enemy, then doublecross them!
The enemy or your friends. Take your pick.
Commentary by memnarch (who has not seen the movie)
Dang, so much for DJ doing a rescue then. That just leaves BB-8 managing a rescue and Holdo causing enough of a distraction with the ship crash. I can't see BB doing anything that'd keep Rose and Finn from simply getting shot, so that crash better be really impressive.
Now the big question is, why would the First Order let DJ go? DJ's already done one betrayal; what would stop him from doing that again? The First Order is made up of, or at least led by, a bunch of evil people that'd have no problems simply killing him as well. DJ doesn't even have the excuse of being able to lead the First Order to the Resistance like Han and Chewie did as Crait is right there. Perhaps if DJ can pull another betrayal out of thin air that'd work, but he'd still be left with an unfriendly Rose and Finn.
Transcript
Rose: Wait a minute. The troopers shove me and Finn to the deck?
Rose: Where are BB-8 and DJ?
GM: BB-8 managed to avoid detection. DJ, on the other hand...
DJ: I cut a deal.
GM: Jim passed me a comprehensive note.
DJ: I radioed ahead and told them you’d be infiltrating the computer room. Money is all I love, so that’s what I’ll receive.
Finn: You sold us out?!
DJ: Omnia cum pretio. I was in prison in Canto Bight for a reason.
Rose: This is crazy!
DJ: No, actually it’s a cool plan I had to get the cash we need to make all those orders to tie up their fleet!
Rose: We didn’t need the money! We were going to hack the orders directly into their system!
DJ: Every plan needs a good backup plan.
Rose: “Good”??
General Phasma: This discourse is very interesting.
Rose: We’re not Dark Side. We’re allowed to have out-of-character conversations!