<human>: Can you give me the gist of the text in a nutshell?

I've been living with my girlfriend and her daughter for nearly two years and love them both very much. The BioDad or his parents split time with the daughter and I have only seen him once in all this time, never spoken or interacted. I wouldn't consider myself a step-parent simply because we aren't yet married, but we live in the same home four days a week, and at times I have to act as a disciplinary figure. I am very careful about the way I speak and interact with my girlfriend's daughter; I don't raise my voice but I do instruct her and occasionally send her to time out when she misbehaves; neither of us will ever spank, though BioDad apparently does. Last night, my girlfriend received a text from BioDad that read, "If your boyfriend is ever mean to my little girl again I will beat his ass." We've never mentioned any of the random things that have been said by the daughter about her father, like "My daddy says a lot of bad words" or "My daddy doesn't live with me" or "My daddy spanks me." We've always regarded the things she says as not necessarily being accurate or truthful, after all, she is a 3 year old. I'm at a loss now. My girlfriend hates discussing her parenting plans with me and doesn't involve me in any of the family law/court nonsense. I've never actually met the person that has threatened me, but I certainly don't appreciate his attempts to intimidate, manipulate, and control my home life.
<bot>: BioDad is sending my girlfriend text messages that physically threaten me. Do I do anything at all, let it go, leave it up to my girlfriend to handle?