This was a great character moment here. I have to wonder if it was written in the script, suggested by the director, or if the actors initiated it.
I feel very sure that Milady is more than badass enough to mount a horse without help. But she got Athos to help her up. Is it because, in spite of everything she’s done, she still wants to be treated like a lady? Was it another of her mind games, proving she can demand such treatment from her husband and get it? And was Athos’ acquiescence due to a knee-jerk adherence to the gentleman’s code of conduct, or because he still has feelings for her? Or maybe he just wanted to get her conniving butt on the horse so they could go, haha.