An ex-student of my future supervisor is accusing the lab of fraud. How does this affect me?
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.