body odor can actually be the result of a metabolic disorder of which the customer is not aware. This may be the best angle to approach this sensitive subject. Here is a link to an article talking about it http://www.medicinenet.com/script/main/art.asp?articlekey=149212. It is quite possible that the customer is not aware of it because few people have the courage to address it because it is awkward.
Credit to you for wanting to find a diplomatic way.