I have heard of it used with boas however what you need to remember is that the most important is that if you need if you suspect a RI you need to get a culture done to identify which type of RI you are dealing with, after that the proper anti-biotic will be prescribed.
In other words do not decide to use Tylan just because you may have some laying around.
Also based on the vet answer, you may want to find another one, there are some vet that will prescribe things just in case or even without a culture and those are the ones you want to avoid.