It was probably best that you re-homed the boa. They can grow to be rather large, and potentially aggressive, making them a dangerous pet for someone with a houseful of cats and ferrets. And, if you can not commit to breeding and culling your own prey animals for it, you surely can not provide the level of care it needs. Most responsible herp owners will (or at least should) breed thier own feeder animals for their snake, to ensure that they are parasite free and of the maximum nutritional value. And NO responsible herp owner feeds live and unstunned, as rodents can severely wound and kill your snake. I realize in the wild they fair fine, but in the wild, the animals also are not confined and forced to fight to the death; its more of a hit-or-miss kind of system. So most of them have to kill or stun the mice/rats/rabbits/chickens before feeding... which can be ugly (I've had to do it before....)
In short, what I'm saying is, it takes a real reptile enthusiast to own a snake. You have a wonderful compassionate heart, but it is so compassionate that I don't think you'd be able to kill one animal to feed another with a clear conscience, which is fine. After all, snakes are wild animals who don't belong in captivity, so keeping them here has created a market for feeder animals that really shouldn't have had to exist to begin with.
In the future, avoid impulse-buying reptiles. It promotes trades who mass sell them to a public that often does not know about their proper care. Most people do not have what it takes to care for a large constrictor, yet these stores selling them to not care about that one iota. As a result, people and domestic pets have been harmed and killed by constrictors, but more often, the constrictors themselves end up hurt: neglected, abandoned, or let loose to suffer whatever fate awaits them. You hear plenty about the crisis of homeless dogs and cats, yet never a word about the thousands of reptiles who are displaced across the US. It is truly tragic.
In this case, you did the right thing by the snake to the best of your abilities. Sounds like he was being neglected, but hopefully now he is with a herp-knowledgeable owner who will get him into a good state of health. Thanks for looking out for him, and I hope you feel better about the little mouse soon.
I wouldn't have advised feeding a pet shop mouse to him either as you have to be careful they dont have diseases or anything.
How can they eat the things they do whole though. 
