I think my two examples crossed paths in a way they weren't supposed to. In both situations I want to respect others beliefs however...
In Example B where my friend prays silently, she catches me off guard and I feel uncomfortable in that I feel like I am doing something inappropriate (talking, chewing, etc.) while she is saying grace. I guess since she has no issue then I should have no issue with it. I just wish she would give me a heads up so I can act respectfully during her prayer.
In Example A, this is really more the intent of my post. I have no problem with others saying grace for themselves at my dinnertable or in my home. I do have a problem with being expected to join. In fact I have never once been asked, just expected to go along with it.
As far as the family praying in your home like that. They probably felt that since you did not initiate the prayer they would for the son's sake. If you had never eaten with them before I'm sure you would be surprised, but you could say your own silent thank or whatever while they are saying their prayer. Or do a moment of silence.
I would not be too upset, since they probably didn't intend to offend you. Maybe the next time before you eat you could let them know that you feel uncomfortable with it. I tend to let things like that roll off my back.