What would you Do?

This is not a pregnancy question but technically it is related to reproduction! 
What would you do if you knew a very close family member's real father and she did not?
Basically, Susie does not share a father with her siblings. Her siblings' father died years before she was born. Her family used to live in Europe and moved to the US when Susie was very young. So people back home in Europe who knew her siblings' dad know he is not Susie's dad. But in the US only family knows. 
Susie's mom gave her the same last name as her siblings and told her she was pregnant with Susie when he died. Susie is 30 years old and still does not know that man was not her father.
Her real father knew about her but left her mom and was never interested in her in any way. He never asked about her or paid child support or anything. He died more than 10 years ago. He is not listed on her birth certificate.
Should anyone other than her mom tell Susie  about her dad? Her mom has decided to take it to the grave if possible. Knowing her if she is told she is going to be extremely upset, depressed and I do not think she will ever forgive any of us for lying to her for all these years.
If you could guarantee me she would never find out I would totally be against telling her. The man is dead just like the man she thought her father is. He never cared for her or wanted her so what good would knowing that do? He really had no family either so it's not like she has other relatives dying to connect with her.
But I worry that one day, someone who knows her siblings' father will say something. It has been 30 years so far and nothing.
What do you think? Tell her? Don't tell her? 
***small edit***
The only reason I thought it may be my responsibility to say something at some point is her mom has been really ill and may not make it.
Susie rarely talks about "dad" but she makes comments sometimes. As long as her mom is alive I feel it is none of my business but I worry that if her mom dies and she ever finds out since we are so close she will never forgive me. Anyway, I will just take the chance that someone may tell her. I hope not but there's no point in making her feel unwanted.

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