I feel like it's quite possible that this is someone we know, or connected to someone we know.
The action's name is Witch Hunter, not Gretel, which is odd because typically we don't get that much in the way of 'generic' social actions. Usually social actions are about a specific named character, a group, or about a given topic. When it's the former, it's usually something they're wrapping up or just starting; whereas in the latter examples, information is obtained more directly. We hardly ever get social actions to handle someone else's problem for them.
So it's odd to focus on the Witch Hunter - I'd assume it'd normally be named Gretel and that it'd be about Mathilde backing her up in telling him that he's barking up the wrong tree. My bet is that this is Roswita's sibling.
...I suppose it's also distantly possible that Gretel has been turned by a Lahmian or something but I'd hope not.