You both stop her and then explain the situation.

Your girlfriend/Mrs. Bradley grabs Claire before she can make for the door and your girlfriend shudders and shifts. Her fit MILF form shrinks leaving her clothes loose and her red lustrous hair becomes brown again leaving your girlfriend standing in Mrs. Bradley’s half filled clothes.

“Whatta? It's you! Mom? Who the fuck are you? What the fuck are you?” Claire shouts as she recoils from her friend.

“Claire, it’s really me, your best friend. I know, what you just saw is absolutely insane and I’m gonna sound crazy, but I can explain, I’m a shapeshifter.” your shapeshifting girlfriend says attempting to calm down her friend.

“Okay then, prove that you’re my best friend and not some bodysnatcher. Tell me a secret I only told you.” Claire says hesitantly rising to her feet.

“Two years ago, all around jock Alex Fields had a crush on you and when he asked you out, you gave yourself a panic attack because you somehow never saw it coming.” your girlfriend says rolling her eyes as if this were common knowledge.

Claire loosens up a bit and gives Sarah a wide eyed stare, “Oh. My. God. It really is you.” She says as she touches your girlfriend’s face in awe. “First, how are you a shapeshifter? And second, WHY DID YOU TURN INTO MY MOM AND THEN SCREWING HER!” Claire says furiously at the both of you.

Having gotten dressed during the girl’s conversation you respond, “Well for one your mom is seriously hot and with my girlfriend’s shapeshifting powers I really couldn’t pass up the opportunity.”

Claire gives you a look of utter disgust at your response while your girlfriend says, “I don’t know how I got them. I just know that I was sick a couple months ago during highschool and then after I got over it I thought about my mom and before I knew it I was her. I tried with other people including gender, but except animals. And I finally mastered it a few days ago. Now Claire, I need you to play it cool. You cannot tell anyone. Who knows what the government or someone would want with a shapeshifting girl.”

“So what’s the plan?” Claire asks.

“I need you to go get my clothes from your house and I need you Claire to stalk anyone who might arrive until Liam gets back. Then we get out of here and go test out this shapeshifting or something. Sound good?” Sarah asks.

“I will say I’m a bit curious about this shapeshifting so I’m in.” Claire says nervously.

“Anything for you babe.” You say as you give your girlfriend a kiss and head for the door.