Hi
This is going to sound daft but bear with me.
Can you held the snake near your ear and move down its body listening to find out where the sounds are coming from?
If you hear clicks, pops etc from the lungs then it is pretty definately an RI. Whereas if you only hear noises at his nose it could simply be the shed.
How much would the vet charge to get a culture done? If it is something that moves quickly it might not hurt to be one step ahead - and you can't get a reliable culture after you have given anti-biotics I think ( someone correct me if I'm wrong about that though ).
I'm sure I have read somewhere that most RI's and from bacteria that exist in the snakes system normally and cause no problems but flare up if the snakes immune system gets compromised by stress or low temps and such. So moving to a new environment could trigger it on its own if the snake reacts badly to the change. Anyone else remember reading this?
So the snakes wouldn't necessarily need to have come into contact and could have a different root cause for the RI - that's why the culture is the best way to go if you can as it should make sure you are giving the most effective treatment you can.
Also, since it is important to finish the full course of treatment, your BF would
still have to go to the vets to get the rest of the course wouldn't he?
dr del