That is a real shame, but perhaps he has reasons for not telling her. She would be late 70s wouldn't she if I am remembering correctly, but we don't know the state of her health. I suppose it might come as a shock to know that she has a full blood younger sister, who for some reason was "given" away. It does make you wonder if Carl Colson knew doesn't it, as it was kept such a secret? The elder sister was obviously old enough to remember Carl coming into their lives, hence knowing he wasn't her biological father, but wasn't old enough to remember when her sister was born.
I do seriously wonder whether Allan Neill was actually your lady's father. Could her mother have had an affair while Allan was in the army? She could easily have put his name on the certificate and that would have been accepted as she was married to him at the time. That would explain why she gave the baby up and didn't try and claim her back, or mention her when she went to America.It does seem very strange to me that if she was just given up because she couldn't cope, that she was never mentioned. The only way to find out for sure would be for both parties to take a DNA test, and I can't see that happening.
I'm afraid there isn't a lot you can do though if the son isn't willing to tell his mother.
It might be a good idea to post your last request re finding Allan Neill's new family on the Scottish board, but with a link back to this thread so people don't waste time going over old ground. The only thing to keep in mind though, is that if my suspicion's about your lady's birth father are right, any children of Allan for that marriage won't be connected to her.