I highly doubt it's something like IBD, as I was under the impression that IBD was a disease that's tell-tale sign was neurological problems and strange movement. The first thing I would think to try is to soak him/her in warm water for 45 minutes; maybe they just need to go and they can't. The other thing I would do is STRICT Q/T, make SURE to wash your hands well and disinfect ANYTHING you use on the BRB. You might not even want to handle YOUR snake before you find out what's wrong with your friend's snake or until they come pick him/her up. Relax and try to do some research on the web too, especially on other snake sites as well.
IBD in boa constrictors is: "First signs may include off and on regurgitation, and some develop head tremors. Dysecdysis (abnormal shedding) may occur. Some develop chronic regurgitation and anorexia (they stop eating). Not all infected snakes regurgitate, however. Boas lose weight and may develop clogged nares (nostrils), stomatitis or pneumonia. The disease can rapidly progress to nervous system signs, such as disorientation, corkscrewing of the head and neck, holding the head in abnormal and unnatural positions, rolling onto the back or stargazing." It doesn't sound like s/he's doing any of that, so that's good.
Crypto had a possible symptom of swelling of the abdomen, but it sounds like regurgitation is a key sign for it too. If s/he's eating and not throwing up, I'd tend to lean more to the fact of him/her either having some sort of impaction or such. Do you feed on substrate? If you do, they could've ingested some of it and it's causing a blockage. I would move him/her onto paper towels as well, it will help you see better if they pass something, or if s/he does go, it will give you a good sample to keep for your friend to take to the vet.