'Selling Sunset' Agent Mary Fitzgerald Cool with Chrishell Dating Her Ex

"Selling Sunset" star Mary Fitzgerald is throwing girl code out the window ... she's got no problem with co-star Chrishell Stause dating her ex, who just so happens to be their boss.

Mary joined us on "TMZ Live" Tuesday and we asked her about girl code on the show, or lack thereof, as it relates to Chrishell continuing to move on from her divorce by getting with Mary's former flame, Jason Oppenheim ... who owns the agency featured on the show.

As we reported ... Chrishell and Jason went public with their budding relationship back in July, on a European group getaway with Mary and her husband, Romain Bonnet.

Mary says she was actually one of the driving forces behind Chrishell and Jason announcing their romance to the world ... so she's definitely putting her money where her mouth is here, telling us how she egged them on to go Instagram official.

Mary and Jason dated a few years ago, but remained close ... and she tells us what advice she gave to JO when he started showing an interest in Chrishell.

While the relationship has Mary's blessing, it's still too fresh to be featured on the upcoming season 4 of "Selling Sunset," which drops Wednesday on Netflix. She says that'll be on season 5.