Do you have contact details?
If so, I'd message: 'hi Sarah, we were all in a rush today so I didn't get a chance to clarify, but I definitely can't ask poor Adam to commit to walking with Bobby to school. He's usually hanging out with his y9 mates, & he walked independently from y7. There's all sorts going on afterschool, too, so Bobby would be on his own on the way back anyway.
I'm sure Bobby will find his feet!
All the best,
OP.'
If no obvious way of finding mum & saying this, I might treat my ds to a lift/taxi for the first week.
This puts the onus on her to either accept the situation or find you to discuss it, at which point, response as above.
Obviously you should not have to, but my concern would be that otherwise her y7 just tags on. It's not HIS fault.
& then either you're saddled with the arrangement, or you/ds have to pointedly & explicitly terminate it. Mean to the y7 with a CFer mum, & awkward for your ds.