It would heavily depend on context and the situation. And in addition to the other replies, I think it would differ based on if Frieren was alone or traveling with someone, and who that was.

If she and Fern met them at a resturant, things might work out. But I don’t think there would be much left of them if she met them alone on a remote path. At that point she’d probably attack first and ask questions later. Although she might hesitate to harm him in the process. So yeah, it depends on how they meet and if she or someone else with her can detect the difference.