@andromeburns
Imagine your friends dilemma.
Telling you that your mum had an affair, fell pregnant and gave the baby up for adoption.
What would you have done, fallen out with your friend, shoot the messenger scenario, confront your parents, cause a massive scene, been hurt to the core and in turn hurt your DM, would you have believed your friend, would you have felt your friend was spreading false accusations about your DM?
Once your DM had passed it would have been even more difficult to tell you once your DM was no longer around to tell you the truth.
Your friend was put in a horrible position, im not surprised she kept quiet, she should have never admitted she knew but I suspect, after carrying this secret for 30 yrs, she was probably glad you knew.
Its not a scenario I would have liked to be put in, I think your friend was caught between a rock and a hard place.