I am no expert but I don't think they helped their case in that email when they said that they wanted the domain to extend the writings of the daddyissues__ twitter feed to "entertain people as my hobby" rather than be straight and tell you that it was because they owned the daddyissues clothing brand.
Squirtit has done you a massive favor by connecting the dots here. Kudos to him.
Also, her twitter feed
Daddyissues (@Daddyissues__) | Twitter (2 underscores) may be quite funny in places but if she can't even get
Daddy Issues (@DaddyIssues) | Twitter or
Daddy Issues (@daddyissues_) | Twitter (only 1 underscore) then how on earth can she think she can just take your domain?
Somebody should register daddyissues__.com
and offer to sell it to her.