It might totally depend on the contract between the employer and the leasing company as to whether. I know ours specifically stated the relationships for specific laws. Your leasing company should be able to answer this (unless you are the leasing company...then I would seek legal advice outside what can be provided for free on an internet forum).

I would suggest reading:http://www.hhs.gov/hipaafaq/providers/covered/499.html because you have HIPAA in your title, but not in your message, so I am guessing that is what you need to know this for.  The health plan entity is covered by HIPAA, but it is my understanding that the employer itself is not unless it is the one dealing with the actual claims which most normal employers do not.  What type of health plan does A have?

I found this link later this evening:
http://www.hhs.gov/ocr/privacy/hipaa/understanding/coveredentities/businessassociates.html as I was still curious.

It doesn't sound to me like B would be a business associate under HIPAA unless A and B are in a regular environment where HIPAA would come into play.  Do you mind posting what their primary lines of business are? Do you do servicing for providers or health plans?  THe link I just posted gives some really good examples of what a business associate would be.

Just being an employer with health insurance questionnaires who has another firm process them would not make either of the companies fall under HIPAA by itself.  Now if that other firm is processing claims, reviewing actual claims, etc. Then yes, they could be a business associate.