As long as you don't get spiteful, maybe this is good. It shows him that the kids will be an issue to contend with. Problems you may already know that could arise (+others?): - he blames you (hates you), sees it as proof that D is the best - he gives up on them - he loves his kids enough that he slows down the separation - through your/his encouragement, kids see this as a way to show love for him that is unconditional --> who knows...?
I can see why he'd ask you. If you ending off having main custody, the other parent is the loser. For a guy to admit that problem and come asking for help must feel very shameful...
You could set a rule saying that you need to get a job/hobby/shopping/etc done and that you do it when the kids are with their dad, so they don't have a choice. It teaches them the lesson that they'll carry into their marriages, that love means sacrifice and making tough decisions. They could also get a lot of other positive and not so positive lessons.
