If you have kids OP why not you know gently, subtly introduce the idea she can be a great bad "friend auntie" influence to your kids?
Or be a great auntie to her sibling's children?
You don't have to be a parent to be a strong, fun, supportive influence in a child's life.
Yes the comment "that's life" could be interpreted as very uncaring but in reality that is the situation with your friend right now, and with many others.
Some people don't long for children but a house by the sea, with a sea view and it can be a longing, but that doesn't happen for everyone, some people want to I don't know see The Great Wall of China or whatever, there are so many things that people set their life goals around and sometimes, a lot of times, that doesn't happen.
I agree that wanting a child, to be a family, to have that unit is the most complicated of the longings, probably the most basic human desire, and also probably the most complicated of desires to yearn for and it never be achieved.
But there is a fine line of acknowledging someone's sense of loss and sadness and it becoming something that defines them, they never move on or heal or accept and it becomes their defining feature, they tear up when others innocently mention their family or having grandchildren or whatever.
So whilst you think the "that's life" is cold it can also be part of the balance that people need to move on.