I'm probably going to be told I'm stupid for this, and you maybe right...
But:
I suspect the feet of his ladders are backed up against a concrete foundation for the metal railings; so his ladders are not going to slip backwards.
The right hand side of his ladder looks like it's leaning on the sill, but also against the right hand edge of the stone window opening; a part of the building; and if he knows what he's doing, he'll angle the ladders slightly, so it can't slip left; and definately can't slip right, because of the building infrastructure is stopping it.
I also guess, because we can't see, that he's lifted his ladders up quite high, past the sill; so has a firm handhold.
Okay, his use of ladders isn't text-book; but I think it could be safe.
Please line-up, in an orderly cue to call me stupid!