You are being a bit unreasonable. I understand the worry moving into Secondary. We are so anxious for them to settle and be happy.
I have 2 in Secondary. A dd and a ds. Neither knew anyone when they started. I did worry, and it took plenty of time to settle and find their feet. They took the bus, knowing no-one. Believe me, the school bus is a baptism of fire!
Don't forget this boy is probably trying to find his feet too, no matter how confident and popular he seems. Someone may have teased him about waiting for her? His mates may have asked him to walk with them and he just did? I know it's not too polite but they're 11, at a massive new school trying to fit in and not have attention drawn to themselves. I don't think it's a personal decision against your dd, just perhaps a bit selfish and unthoughtful?
I agree that the bullying by y11's would still have happened if he was there and probably worse,
Don't say anything to the boy. Let it all play out and let dd find her way.
It's hard letting go from Primary to Secondary. You do need to at least give the impression of being relaxed and unbothered, even if you don't feel it. Let your dc tell you their worries but don't act (unless they're serious issues!).
I know a few parents from the time my dd was in Y7 who were ridiculously over-involved in their dcs social life. As a result their dc's seems more immature than others and didn't settle well further into the year as other dcs were uncomfortable with them.
Only now my dd is in Y10 that some of these dcs are becoming more independent. Dd long since moved on to friendships she felt less stifled and 'watched'.
You're dd will be fine in time. She will take her cues from you. Don't panic if there are tears over something minor. It's how you handle it that matters.
Hope things get better and more settled for you both.