I read a book by an advise columnist once. At the end was a chapter called "Questions No Asked". This is sort of the same. I have been around the block a few times, got some things
to share, so here goes.
First, note the title; "If your husband asks...". If in fact it is your boyfriend who asks, say you will have to think about it, excuse yourself, leave quietly and never speak to him again!
1. Assuming your husband's best friend is a horny bachelor; is this going to be a regular thing? How about his second best friend? If he does this nice thing for his buddies they will be obliged to do him nice things. In other words he is whoring you out. Finally if it is a one time thing for the best of reasons, be assured that some day your husband in a moment of anger will point out that you slept with his best friend.
2. If the best friend is in such dire straits, your response could be why doesn't your husband take care of his needs. If he things the suggestion is outrageous, remind him that his request is outrageous. Besides, this is a good way to test the waters for consideration three.
3. If it is a; one time thing for the best of reasons, the only way you should agree is if your husband is in bed with the two of you. That way he can never throw it in your face that the two of you slept together. Make sure you are respected during the festivities and note whether you or the best friend gets the bulk of your husband's attention.