An ex-student of my future supervisor is accusing the lab of fraud. How does this affect me? I start my master's degree in this lab in September. After accepting my offer I found out that an ex-PhD student has been spreading rumors that the supervisor is publishing fraudulent data, going back many years, basically suggesting her whole field of research is a lie. The ex-student was very insistent to let me know about this and encourage me to not go into that lab. I think he is even trying to get an official investigation into her research to get the paper(s) retracted. 

I'm worried that this will affect my career in academia, especially if the research for my master's thesis is based on falsified data. The ex-student apparently got his career ruined as well by the supervisor because he refused to put his name on the paper(s) and she made some calls that resulted in him not getting a post-doc. But I am only hearing one side of the story and I'm not sure what to believe. She is pretty well-known in her field for her research and from my personal interactions seemed nice. I'm scared if I ask her or her other students about it directly it could ruin our relationship before I even begin working there. 

From my understanding my acceptance into the master's program is to the department and I could request another supervisor, though this would definitely make things awkward and there's probably limited options at this point for profs that can accept me. I'm not sure how I should proceed.
With no further information available, this seems like a situation to avoid. If you have time, and if the resolution of the case seems imminent, then you could wait, but otherwise you would be good to protect yourself. Protect yourself from the turmoil that seems inevitable to come as well. 

One way to proceed is to go to the head/chair for a sit down and ask for advice. If the advice is to stay with this professor, you want some assurance that you will be allowed to succeed even in the worst case for the professor. 

The charges might be false, of course, though the professor sabotaging another student would be equally serious, if that is what actually happened. But make sure that you aren't just being driven by rumor here. Make sure you have accurate information. 

The head or another senior professor is likely to have better information.