If they understand how you will be putting the child to sleep and don't have problems with it, I'd be willing to give it a shot, maybe put a clause in his contract about a reasonable adjustment period, and let them know that he'll only have a short period to adjust.

Unless you're having trouble finding new clients, and the extra 1 1/2 months of advertising would be beneficial. In that case you might want to consider having another talk and seeing if she'll start adopting some of your strategies at home instead, and if they're not willing to try to work with you, skip them all together.