My experience isn't quite like yours, but a few years ago, one of my aunt's (dad's sister) received a letter from a woman claiming she was the daughter of their mother (my grandmother). She had been put up for adoption after she was born and had been an only child all her life. In her 60's, she realized she needed more info about her biological family for medical reasons. My grandmother and my grandfather have both been gone for a very long time as well as my dad's oldest half-brother (from a marriage my grandmother had had before she met my grandfather). This woman only had my grandmother's name on record. I don't think she knows who her father was, and if my grandmother knew, she took it to the grave.
Anyway, when my dad and his brothers and sisters found out about the letter, they were unsure of what to do. There are a lot of them (five sisters, three surviving brothers). I think there was a lot of confusion about why their mother had never mentioned this half-sister, since they'd always known their half-brother before. They decided they'd go ahead and meet her in a neutral place to make certain she was who she said she was. They met at a restaurant, and my dad said that as soon as she walked through the door, it was like their mother coming back to them. They immediately embraced her, and she's been a part of our family since.
I don't know everything about your mother's situation, but it might be a good idea to just mention that you met this woman. Then, leave it to your mother to take the next step, if she wants to at all. Sometimes, good can come from reuniting with family. My own mother hasn't seen her brothers in a long time, and she feels so hurt to be left out of the loop, discovering from someone else that her one brother has cancer. If this relationship would only cause more harm, I think it would be up to your mother to decide whether or not to pursue it. Good luck!