How Does She Tell Her Sister This Secret – OPP

A woman knows something about her sister's new boyfriend. She can't tell though because she would lose her job. But she is worried about her sister getting hurt.

Mike & Kate-I have a problem. My sister has met a man she claims is the guy of her dreams. They have been dating for about 3 months and she finally decided that the family could meet him. Last weekend my husband and I went out to dinner with her and this new man. Immediately I recognized him but couldn't place him. I asked him if we'd met before and he said no and immediately changed the subject.
My sister is in her late 30s and has never been married. She is so excited about this guy. She feels she's finally met a quality guy. I've never seen her so happy. And he seems really into her and a very nice guy.
Two days after we had dinner it hit me how I knew this guy. I'm a nurse at a urologist office and about 6 months ago, this guy came in an got snipped! Obviously because of HIPAA, I can't tell my sister this. I did ask her if they've talked about what they want in the future, knowing how much my sister wants to be a mom. She told me they are both on the same page. They both have the same desire to be married and have kids one day. She said he had been married before.  I'm not sure why this guy would tell my sister he wants kids when he can't have them.
I'm torn as to what to do. I don't want my sister to get hurt. She is falling for this guy big time. And she believes they have the same idea about the future. If she continues seeing him she will only get hurt when she finds out he can't have kids. What if he never tells her and then they go through fertility struggles and the entire time he knows it's him? But if I tell her what I know, he could have me fired for a HIPAA violation. Is there a way I can let my sister know about this situation so she could find out the truth? I only want her to be happy.

Thanks for your help