With you doing the leading, have your husband behind the mare with a feed sack (empty of course) that he will "rattle" when she lags behind where she should be, which is with throatlatch at your shoulder.
He needs to not rattle bag unless and until the filly lags behind, stopping the INSTANT she steps up to where she is supposed to be.
Walk briskly, and with little talking, as she needs to concentrate on moving out, not listening to you. Don't dawdle, but walk at nice pace. Halt her, and move her forwards several times, until she steps out nicely when you lead off.
Again, he only shakes bag very lightly, and very low, not over his head, or waving around like he is landing fighters on aircraft carriers. And ONLY when she is needing to be reminded to step forwards to be at your shoulder, and he stops the instant she has moved to that point.
Very quiet cues, very quiet movements here.
Horses make me a better person.