If you pull out of an arrangement to share a house at very short notice...

[BertrandRussell]

....do you have any responsibility to the other person?
My friend's dd was due to move in with her boyfriend on September 1st. He has changed his mind, but she still has to leave her flat.

When you say she still has to leave her flat, has the lease been signed up for someone new already?

“I would save my upset for my own offspring”

Fortunately I have compassion to spare.

Yes- she has no option but to move. New tenants moving in- it was a student rent.

As she didn't protect herself legally then she doesn't have any recourse.

Sounds like he got cold feet. Nothing she can do but find a new place asap and learn from the experience.

No legal recourse of course. But you would have thought he’d at least offer to let her store her stuff.

Oh well, as pp have said, she won’t make that mistake again.

Some people just have more empathy than others. Nothing wrong with getting upset on behalf of strangers.
I guess the OP feels her friend wasn't looking out for her child properly.

But you would have thought he’d at least offer to let her store her stuff.

He must know what impact this last minute decision will have on her so doubt he gives a shit unfortunately. I do hope she tells him where to stick his suggestion of being 'friends'?