How long have you two been together? What makes you think she would cheat on you? Is it something she said? Does she fancy this friend? Are they lunching everyday?
Based on what little info. you gave, I would say, you shouldn’t get worked up about this and make a big scene.
But it’s ok to be jealous occasionally, ’cause it reminds both people not to take each other for granted. Chat to her about it. 🙄
Smokey.