He can't. He'd have to turn her over to Uriel. And turns out it's good he didn't do that because Uriel was going to annihilate her.
But I like your sneaky way of thinking.
There is a crack, a crack in everything
That's how the light gets in Cohen

Lucifer is in charge of Hell. He could send her back, but not as a prisoner. Or be a smart ass and send her back, then allow her to come back.

He loves his motherand is reconciling with her. What son would do that to their mother?
Plus, he finally admits that he doesn't know what God wants him to do, just showed him her empty cell. Everything else has been based on assumption.
Uriel believed God would forgive Mother. This is all, somehow, part of God's "plan".
There is a crack, a crack in everything
That's how the light gets in Cohen

When Lucifer made his deal to send his Mother back to Hell, he thought Amenadiel would be able to take her back to Hell. Before that could happen, Amenadiel lost his powers. After that, there was no way for Lucifer to do anything but allow her to remain on earth, even though he did not know that when he passed sentence on her.