It isn't as if she offered it because she wanted to. Lethe, too, is displeased by the situation forcing her to the side of someone she neither likes nor trusts; but she's done it before.

She does it clumsily. Her movements aren't synched with his, her fighting isn't measured to enhance his. They are two people attacking the same enemy, but very disconnected. It can't last this way for long, not under that rush as the enemy hurries towards them.