I know that when my father died, seeing people from his past there, people I hadn't seen in decades made an impression on me and brought home the point of what a wonderful person he was and that he had touched people's lives so much even back in the '60's before I was born, that they paid their respects to him by showing up at the funeral or the showing. I know they weren't there for me, they didn't even know me or my mother. They were there to show their respect for my dad.
I think you should do the same thing. If I were you, I would even go through the line and express your condolences to the widow. No need for a lengthy conversation, just tell her you loved him, and you miss him. No witness in their right mind would have a problem with that. As for having to listen to the talk, who cares??? It's what HE BELIEVED!!! You don't have to believe it, but it is being given because he would want it to be given.
I hate to go back to my dad's funeral, but I had put into the memorial pamphlets two scriptures. Daniel 2:44 and Rev. 21:3,4. Not because I wanted people to know what the Witnesses teach about it, but because HE would want something said about God's Kingdom and something said about a paradise earth......whether it's true or not, it doesn't matter.
I appreciate your problem here, but I say go for him. You will only regret it if you don't go.
I am sorry for your loss. May our Heavenly Father give him his reward.