Some people on here have children with allergies, who can get shots for it. Others simply take antihistamines.
Personally, well done on taking this lady in, and don't give up the good work. You are clearly who she needs to be with. Can your bf stay somewhere else until she has been cleaned up and is back to 'normal' Bengal condition? It may only take a week or two if she has settled and calmed, as her stress will be having a big impact on her symptoms.
What has your vet said? Can he give you some wipes to help lessen the dander from her coat for a time? A damp chamoise leather is also good for doing this, and it reduces the odd smells from the coat.
Alternatively, can you stay at your friends for a couple weeks while your bf settles into the flat and you look for the solution to make this work?
I personally would say to heck with it, and find somewhere to be crazy cat lady on my own, if it doesn't improve with your bf.... Poisoning is a bit extreme if a cat allergy is his only problem!