Death of spouse. Father has dementia and doesn't get it. What do we tell him?

If your father is in the late stage of dementia, he is far beyond the ability to get it. no matter how many times you discuss, he will not remember. He is simply not able to remember that it happened, or remember that you told him again and again that it happened. In his mind he may be 50 years old, or 20 (as he has lost the most recent memories first) and he expects his wife to be alive. Reminding him causes him to grieve all over again. This is the time for therapeudic fibbing and redirection. He asks where is mom, you say she is shopping, or visiting her sister, or whatever she would have been doing years prior when she was more active. Then you redirect the conversation another topic such as looking at old photos, or having tea or a snack.

Telling him his wife passed, when he has no ability to remember and will have to process it over and over again is a cycle that will never end.

Angel

What do we do about funeral? Take him to socialize with family

I believe he should attend the funeral. During the funeral he may realize what is happening or he may not. He may understand what is going on, or he may ask. At the funeral I believe you can be honest about what is happening in the moment. He says ... where are we, you say, we are at mom's funeral. He may socialize with family or he may not. But I have no doubt that the next day or days, he will not remember that he attended, and there is no reason to remind him that he attended. His mind simply cannot create new memories any more.

Angel

I'm sorry for your loss, dealing with FIL must make it doubly hard. Yes, I believe he deserves to attend the funeral, it may even help him to understand that she is gone, at least for the moment. You might consider having a friend or caregiver look after him so the rest of the family can take part without worrying about him acting out, he may even need to leave early, and that's OK too.
Aging Care has an article that may help:

www.agingcare.com/articles/telling-someone-with-dementia-their-spouse-died-133806.htm

Personally I see no valid reason he should attend the funeral.
He will be overwhelmed and possibly frightened at the number of people, the noise, the confusion as well as the length of time will be exhausting for him.
Have someone stay home with him and keep his regular routine and surroundings.
As to what to tell him..
1.Mom went to the store
2.Mom went to the Doctors office
3.Mom had an appointment to get her hair done
4.Mom went to lunch with her girlfriend.
Then repeat 1 through 4 as often as necessary.
Do not tell him she died or passed away he will relive that death as if it is the first time every time.

If you are planning a lunch after the funeral and if it will be small he could attend but if there will be a lot of people again I suggest he stay where he is comfortable. A few people can come by and visit later but keep the visiting number to a few and only for as long as he can tolerate the visits.
Good luck.