My girlfriend's mom refuses to accept her as gay and our relationship

My girlfriend (let's call her Sam) and I started dating at college have been together for almost a year now (still at college). My family is SUPER accepting of my sexuality and our relationship...her's however are not so much. Her mother is toxic to the point of calling Sam fat and unable to get a boyfriend because she's physically unattractive to men. Her mother found out about us dating and went on a HUGE tirade about how Sam is a sinner, a disgrace to the family, not allowed to be gay, and not allowed to return to where we go to college together (which was a bluff). Some background, her mother is extremely explosive about things that upset her, but in a day essentially acts like it never happened and is best buddies with Sam gain. Applying her pattern of reaction to Sam's sexuality, her mom consistently BLOWS up and calls her extremely derrogatory names, says her family will never support her, and that I am never allowed to be with Sam ever. After she gets over seeing something she doesn't like such as a picture of us or a text from me on Sam's phone, she acts as if I don't exist. Whenever she stumbles across something about me again, she is startled and emotional and acts as if Sam's sexuality is a horrific surprise every time. The hard part is that Sam and her mother had a close relationship when she was in high school; so being in a position where her mother consistently berades her really hurts her emotionally. Additionally, Sam is financially dependent on her parents for tuition money. These circumstances make a break from her mom nearly impossible. Most recently, I attended one of Sam's field hockey games, where her mother saw me and was visibly livid that I was there. She texted Sam after the game about how she refuses to support Sam while "it" (that's me) is in the picture and from here on out will not come to any of her games. Well, within 30 minutes after the game when I had left, her mom was acting as if nothing ever happened. This pattern is psychologically DRAINING for Sam and also makes it extremely hard for Sam and I to participate in each other's lives at functions like field hockey games, or seeing each other over summer and winter breaks because I basically have to be kept a secret in order for her mom to not explode and threaten to pull Sam out of school and ban her from seeing me. How does Sam conquer this toxic emotional battle with her mother and how do I continue to be present in Sam's life when I have to be basically kept a secret from her mom?