I've never been a husband, but will give my knowledge and beliefs the best that I can. If there is nothing left to the marriage, he needs to come to terms with that and do some grieving for this marriage and yes, the past wife also. It doesn't make things easier to be involved with other people while you two are still married and have a connection between you like your daughter. Such relationships more often then not are simply emotional rescues and don't create a future healthy relationship with the new partner. There is still too much pain and frustration, and even feelings for the marriage split that a new partner simply medicates the pain much as drinking or drugging would do. I have heard plenty of talk shows say that after a divorce you need a period of time with seeing no one new. Time to grieve, and let go, and collect your thoughts, before you move on to someone new. I know life doesn't always work out that way, and it takes great discipline to control our need to feel better through the pain.
Any relationship entered into at any time is going to take a lot of work. But if that desire and willingness to work on it is not there, it must be accepted as such. You can't get back what is no longer there.

Here's a wish to happy future relationships for the both of you!