Was your dh's gran mentally competent when she made her will?
I can understand your dh's feelings, but is your FIL also not going to attend his mother's funeral?
If your FIL is attending, I am of the opinion that your dh should go to support his pa.
If they time their arrival to coincide with the coffin being carried into the church/crematorium, they can leave immediately after the service and should have no need to enter into any conversation with any other parties present if they don't wish to do so.
If they are both of the opinion that they do not wish to attend, courtesy dictates that they send a floral tribute to the funeral directors.
I would suggest that at a later date you, your dh, dcs, and your FIL hold your own private celebration of the lady's life; maybe read a poem or two, drink a toast to her, and send a few chinese lanterns up into the sky or plant a tree in her memory.
An alternative would be to jointly name a star after her or similar of course, you could always do a Steptoe, foot on the gas back to gran's house after the funeral, and quietly remove a keepsake or two