How old is your charge?
I have been in the exact situation. I started when my charge was 12 weeks. He would only nap when he was held; it sucked. I know the pp offered advice about waiting until she is very asleep but we both know that doesn't work. My charge would awake no matter how deep asleep until I picked him up again. I couldn't do laundry, dishes, or even get a lunch break.
I think you are in a tricky situation because it all depends on what MB and DB want. I would talk with them and see what methods they would be willing to try. At first MB was reluctant to try anything but she realized that he was not getting sufficient deep sleep while being held. Eventually we " trained" him. Slowly it went from attempting 3 times, and letting him fuss for a few, and following that pattern. The best thing we did was swaddle. He started sleeping up to 2.5 hour naps at 4 months with his swaddle.
I feel your pain. I was in the same situation and felt desperate too. Just remember it won't last forever. Please talk to MB.