On the evidence we have so far I wouldn't call Mary a co-conspirator in Henry's flirting/seduction schemes, at least not in the way that in Les Liaisons Dangereuses Madame de Merteuil is the equal instigator and co-conspirator with Valmont in their scheme to debauch a virtuous young woman.

I read Mary's attitude to Henry's flirting-as-seduction as acceptance. In the world she comes from this is the way things are - it is what some men men do, different only in degree from flirting as being pleasant or making oneself agreeable.

Edmund, who could no more flirt than fly, is an unfamiliar oddity to Mary - he is "without any of the arts of flattery or the gaieties of small talk, ........he was not pleasant by any common rule: he talked no nonsense; he paid no compliments".

Mary's general attitude to the women whom Henry hits on seems to be one of indifference. If a woman doesn't want to get her heart broken then she should avoid Henry (ch.4) and men like him. We see her indifference when Mrs Grant imagines that Julia is in love with Henry (ch.17) :
“I dare say she is,” replied Mary coldly. “I imagine both sisters are.”

As Mary sees it, both Maria (who is engaged!) and Julia are eager volunteers, practically elbowing each other out of the way to get Henry's attention. If they get their hearts broken that's their look-out - it's not Mary's problem.

Mary does protest when Henry decides to target Fanny:

"I do desire that you will not be making her really unhappy; a little love, perhaps, may animate and do her good, but I will not have you plunge her deep, for she is as good a little creature as ever lived, and has a great deal of feeling.”

There isn't much more she can do really. More vigorous opposition might very well only encourage Henry, and I don't think she would want to risk seriously falling out with him. She knows he is selfish and can't be relied on to support her interests, but he is her brother, they come from the same world, and he is the only man who she can call a friend.