Interesting! Until this post, @Helensburghmiddleagedmum, I had a picture of your friend forming in my head. Roughly, I saw a woman who had well and truly clipped the wings of her sons to deliberately keep them at home dancing attendance on her, The Matriarch Queen Bee. One son might 'fail to launch', but both of them?
But, I would expect a Matriarch Queen Bee to parade her sons in front of her friends, because she'd be so very proud of her achievement. Only, your friend doesn't. In TEN YEARS, "none of us has ever seen her sons or even any photos of them". So my picture of Matriarch Queen Bee is wrong.
I know a couple of people have theorised that the sons don't exist, but for me that doesn't fit either. If she had invented two sons, she'd have made the story a whole lot easier by having them both working abroad, very successful, so busy, I don't know what they do they tried to explain but it's so high flown, blah blah blah. No, I don't believe the two sons are pure invention.
So then another picture formed, based on a family I knew who lived it
. Two sons damaged in childbirth by oxygen starvation (cord round the neck in both cases), the mother keeping them both at home with her because she didn't want them to go into institutions. Both sons were able to work via Remploy, but would never have been able to live independently. The boys/men lived with their parents until the death of their father, at which point the mother realised she had to ensure their future after her death, and they both went into supported living.
I wonder if this is where your friend is now. She has two working sons living with her and her husband/their father, of an age when under 'normal' circumstances, they'd have long moved out. She's proud of them for taking their turn at cooking the family meals, proud of them for working, proud of them contributing to the family finances. But, she's treading water and she knows it. Her sons are never going to live independently, possibly not even want to do so. This is her life, and this is always going to be her life, until she dies.
And then you announce you are going to be moving to another phase of your life, becoming a grandmother. And it brings home to her that she has no new phases to her life, just more of the same. If my guess is right, I don't think she's envious of your impending grandmotherhood. I expect she accepted long since that that was never going to be on the cards, dealing with it publicly by claiming "she is glad she won't have grandchildren as she doesn't want the responsibility". No, it has just underlined to her how her future is absolutely set in stone - more of the same.
This is all just me guessing. I may be very far off-beam.
In your shoes, I'd keep going. Sweep the elephant in the room under the rug as best you can
! Continue to make contact, text her, suggest lunch. Hopefully, she will respond.