If youdon't mind me saying your DH is being a bit of a wally sorry
she is obviously sorry and as we do not really know what happened between them as it is not exactly clear, just that there were insults flying about, so hard to made a judgement but still think that as your Mum is trying so hard to put it behind her for thesake of the family your DH should move on.
I hate all this holding on to things like this, having much the same in my own situation at present with my DIL holding on to stuff and making everyone feel in comfortable and making me feel that I can not visit my son Its all NONSENSE and both your DH and my DIL should Grow Up ..
Lastly he should not get one penny of the money that your Mum has given, really the money should go to your daughter that's getting married as she could probably make most use of it as she is starting out in life but of course that is your call 