Hi,
Second the vet idea wholeheartedly. If nothing else the fact it has regurged (was it more than once?) has had a bad effect on its gut bacteria which makes it even harder for the poor thing to recover.
This is kind of why people say there are no free snakes.They knew it needed a vet and didn't want to pay for it either.
What size is the snake? and what conditions were they keeping it in? will it last a little while in good husbandry conditions without feeding attempts or is it basically a train wreck on its last days?
Also I hope you mainatined extremely good quarantine with your existing collection ( other room ,new snake dealt with only after your own, no shared equipment, extreme cleaning between to prevent transfer of nasties). The chances of getting anything you could get a fecal float on are small sadly by the sound of it.
dr del