Fuck This-
Sounds like borderline behavior. This is one of the M.O.s of borderlines... after they split with you, they will often try to turn all your friends against you.
First, not sure how you let her know you'd chosen someone else over her. Generally this is not the best form... makes people feel like you judge them as inadequate, at least compared to other prospects. Fear of these kinds of scorched earth backlashes are one of the reasons why most women are so vague about their unavailability and usually refrain from telling you "I'm seeing someone now!" if they have any social ties to you + recent dates/interactions with you. Instead you just get "Maybe, I'll text you" or "I'm a little busy right now" until you discover a few weeks later the girl's got a boyfriend. You can do the same thing with women: "Can't make it right now" "I'm tied up the rest of the week" "That invite sounds great but I just can't make it ATM." They'll get the hint, and you avoid giving them a too-big ego blow... they can always chalk up the fact that they didn't end up with you to something else.
Now since you are in that position, she is actively working people against you. That means these people are receiving information from her that says you are a bad person, and (I assume) no counter-information from you. Think of it as a disinformation campaign where the other side doesn't try to correct the record (I don't know if you are... just doesn't sound like it).
If these people know you very well, and don't know/trust her, they'll dismiss her claims. Or approach you about them for clarification.
But if they don't know you, and haven't had their eyes opened to broken female histrionics, they may just take what she says at face value: he's a bad guy, he's so evil, he's such a nasty dude, etc. To combat this, if you care enough to combat it, you need to provide information to counter the information she's putting out there. I'd probably just tell the whole story between you and her and let people judge for themselves. If she wants to air part of her dirty laundry with you, you might as well dump the whole load out the window. Then just message everyone in your network and tell them "Here, read this. It's important."
Most of these types of women don't deal well with having to confront the entire story, laid out bare and honest, in the open. They thrive on being the sole attacker against an opponent who does not attempt to or know how to defend himself. And don't deal well with transparency - especially transparency that makes them look manipulative and vindictive.
Chase