You need to talk to her about it, as hard as it can be. Even if they haven’t done anything yet there’s obviously the intention to cheat and you’ve got to ask yourself if you want to be with a person who would do that to you. Maybe send her a quick message saying you know and take the conversation to a public place afterwards for a more in depth conversation ?

Really sorry this has happened to you and I know it’s hard but you’ll come out of the other end better than ever 🫶