How unusual is it for a baby not to be able to sit up at all at 8 months?

I was at a friends house. My daughter is 7 months, and her son is 8 months. My daughter is crawling, pulling up and climbing stairs. Her son still lays flat on his back most of the time and can only sit up in one of those special seats.

I was talking to her, and found out that he can’t roll over in either direction. He wasn’t babbling, cooing, or showing much interest in interacting with anyone, and worst of all, she would get really excited when he smiled—something most people I know stopped getting excited about when their kids were less than 2 months old. Over all, he is about where my daughter was at 8 weeks old. I’m not even sure if he has full head control. It was hard to tell, since he never sat up.

She doesn’t seem worried about it at all. She says he’s “just chillin’ ” And He’ll do things whenever he wants to do them. He’s just lazy” I know kids develop at all different speeds, but he seems to be MUCH slower than any kid I’ve seen his age.

But maybe I don’t have much experience with babies. Is this slower than average but still normal, or does this sound delayed? It’s one of those things I am too embarrassed to ask her about it, but it’s getting very awkward the more my daughter keeps passing him by. I almost feel like I have to apologize for her showing off.