I don't know if Mary is complicit but she certainly seems careless about the damage he does.

In Ch. 18 when Mary comes to Fanny's room to rehearse she says
of Maria and Henry rehearsing
If they are not perfect, I shall be surprised. By the bye, I looked in upon them five minutes ago, and it happened to be exactly at one of the times when they were trying not to embrace, and Mr. Rushworth was with me. I thought he began to look a little queer, so I turned it off as well as I could, by whispering to him, ‘We shall have an excellent Agatha; there is something so maternal in her manner, so completely maternal in her voice and countenance.’ Was not that well done of me? He brightened up directly.

It's interesting that she nonchalantly tells Fanny that she knows exactly what Maria and Henry are up to, but only tried to smooth things over for Mr. Rushworth, which keeps Henry's game going. I'm not finding the right word, but she seems so self-satisfied. I don't know if she's amused by Henry's sporting with girls' affections, but I would not say that she disapproves.

She thought Fanny shouldn't be plunged in deep, but didn't mind that he did that to Maria and Julia.