What was I thinking going to Daytona when I could have stayed home with this thread? Silly me.
The basilisk would likely eat the blue belly (we're talking blue belly swift, right? Spiny lizard, sceloporus?) You'd be risking parasite transfer from the swift to your basilisk...and if it didn't eat it, their requirements are pretty different, I don't see cohabbing them working.
Back on topic...this is an interesting debate I think. Personally, I'm against it, because I don't feed live unless absolutely necessary. But if you're OK with feeding live then I think it's hard to really kill the OP on this one. Small lizards are part of the boa's natural diet; feeding the gecko as part of a "varied diet" isn't so appalling. If anything my issue is with the original owner dumping the gecko on the OP.
I really hate the "it's bred to be a feeder" argument. When you go to the shop to pick up a "feeder" mouse or rat, and they stick it in that cute little carry box with the smiling kids holding the pocket pets on the side, and the "I've found a home!" on one side and "Somebody REALLY loves me!" on the other...well, then those pet/feeder lines blur. And yes, in case you wondered, I've been sold feeders in that very box.