I am so sorry for your OH's loss.
Funerals bring out the worst in people IMO. I have seen more disgusting behaviour at funerals I have attended than all the weddings, Christenings and birthdays parties put together.
As some pps have very wisely said, the ones that did the least make the most noise.
Its important that your OH knows that he did the right thing by his mum. He has to hold on to this, it WILL help him through the next few months.
If there is this sort of petty point scoring going on now, be prepared for some more in the near future.
Its best be start as you mean to go on. Stay away from the stupidness. Even if you and OH stay away 'in your heads' rather than actually IYSWIM. Its for your own sanity.
I speak from a lot of experience. Even at my DD's funeral there was competative grieving and bickering.
What your OH did for his mum when she was alive is what matters. All the bleating and wailing after someone is gone is meaningless noise if there is no substance to back it up.
I hope the day goes as well as possible.
(I hope I dont sound to harsh. I am meaning to be supportive but it dosent always come over that way online)