IF the original RI was successfully treated, then the treatment isn't really at question, but some other variables.
The most common is husbandry... you probably know how to judge that.
Another thing to ask is what symptoms are you seeing? Sometimes scar tissue from the healing process narrows the nasal openings, and a wheezing will occur occasionally.
Probably an important thing to ask yourself, if there is no other apparent reason for this animal to be getting RI's, is if it's immunocompromised. Being a rescue, you don't really have it's life history which leaves a lot of questions. I think the number one reason for being immunocompromised is viral in origin, but we really don't have a firm grasp on the whole pathogen picture. But to make a point, think of Feline Leukemia in cats, probably IBD in boas, and there are a whole range of viral families that may compromise a burm. (IBD, herpes, lente, retro virus' ect).
Something to think about anyways, and probably leads to more questions than answers.
Ian