Blatant Academic Dishonesty

So yesterday I learned that my lab partner has been providing all of our numbers we measure in lab to another person in our lab class. This person just showed up for the very first time yesterday. Not only has my lab partner been providing data to her, but he's also been writing her name down so she gets credit for attendance.

I have been busting my butt staying late with my partner and I feel kinda betrayed that he's been doing this since the beginning. A part of me feels like I should report this to the professor as this feels like a slap in the face to me. Why am I working so hard to do these labs and she can't even show up to record the data? I don't even need her to help, but not being here until now is crazy to me.

So anyways, if you were in my shoes what would you do? I have only known my lab partner for a semester and the other girl I didn't meet until yesterday. Would you report them and deal with the guilt of potentially ruining two people's college journeys? Or would you just suck it up and accept that a student has been able to copy most of your work (he doesn't give her answers to the questions I guess, just data)? It feels like my conscience is screwed either way, as these impact my intergrity in different ways.

Edit*** I also forgot to mention that the same day she showed up for the first time she had to go and move her car. My lab partner tried to sign in for her when the professor came around and she wasn't there. She later came in and signed in for herself but the professor may have suspicions.