Tell her it’s very common, and she will have perfectly healthy children in the future, Be’ezras Hashem. “
You truly mean well, but you cannot know that she will have other children, and her miscarriage is on her right now, so now would not be the best time to say that. She may not be able to hear it yet.
Just tell her how sad you are for her tzorah, and that you are there for her in any way that she needs you to be. Cry with her, if she needs to cry, take her out of her house for a drive, if that’s what she needs. Let her talk nonstop, or sit with her in silence and let her grieve. Just BE there.
My sister had a miscarriage of her first child, after trying for several years to have a baby. B”H she had three other children, but it did not happen again right away. She was not sure she would ever have children. Every person who said,”Don’t cry, you’re young, you’ll have other children,” meant well, but it was like a stab in the heart. I took her aside with me and just held her and told her if she wanted to cry she was safe with me to do so. She had a very long good cry, and felt a little better afterward.
You cannot make promises to someone – there are no guarantees in life, even when we want to give chizuk. The best chizuk is letting someone know you are there for them in the way that THEY need you to be.