Relative I knew nothing about

Hello everyone and thank you in advance for any advice or guidance. I have been struggling for months trying to figure out what to do since I've never thought I would be in this situation. Someone reached out to me on Facebook claiming to be a daughter of my grandma's. The information That was provided to me makes me think it is legit and that my grandma really did put her up for adoption at birth in the 1960s.. I am struggling because I don't know if I should bring it up to my grandma because It seems to me that she wanted to keep it private and that it was a closed adoption. In my mind if my grandma wanted us to know about it she would have told us or if my grandma wanted to reunite with her then I think she would have located her on her own since she has the financial means to do so. I brought it up to my dad when it first happened and my dad says to leave it alone because all they're trying to do is get ahold of her money and he doesn't believe my grandma had another child... I have been contacted by the daughter and the husband of this person and they seem like really nice people but I'm struggling on what to do . I don't want to bring to light my grandma's secret because in my mind it's her secret to keep and to share if or when she is ready but seeing how she is 83 I dont believe it will ever come out.. My heart is torn because I want to do the right thing but I don't know what the right thing is so any advice or guidance is appreciated more than you know. Thank you for your time.