When using the Support Desk function in game to report Harassment, it asks for the character name and server the character is on in the form you fill out. I'll say this though: they will not take any action if you're reporting harassment FOR someone. You have to be a target of the harassment or they say there's nothing they can do. At least, that's the response that I got. Your friend needs to report them, and the person they're targeting with their Party Finder messages needs to report them.