Personaly speaking, i will just change it to Mr. Alan Rud and the late Mrs. Sue Rud, just for formality sake and besides just giving your soon to be son-in-law's mother respect for her demise, although it will be costly, i do think it is the right thing to do because weddings are once in a lifetime.... or do i rather say... Should be once in a lifetime....Etiquette-wise is the right thing to do...

It may be the proper thing to change but if I was the groom, I would want it left Mr. and Mrs. as a tribute to her. She knew about the wedding and probably helped in some way. Another thought, ask your son-in-law to be how he feels and ask his father how he feels about it. Those are the two whose opinion matters the most.

The best thing to do in this situation is check with the son-in-law and possibly his father, it is their decision how they would like her referenced on the invitation. You said that the invitations had been ordered, any good company would be willing to make the changes quickly given the circumstances and it isn't too difficult for them to make a simple adjustment.

This answer may seem to be very blunt but that is not the intention. The simple fact of the matter is that you cannot invite a dead person to a living event (unless you are dealing with the supernatural). It does not matter if you soften the blow by altering the invitation card to say ‘and the late Sue Rud’. Change the card immediately to Mr. Alan Rud and refrain from sending any attachments with it. Mr. Rud will be the first one to be aware that his wife is no longer with him. R.I.P.