This will impact the friendship.

Even if the (well, let's call her "sad mom") sad mom WANTS the friendship to go on as it is, it won't ever be the same. At least, not until maybe someday when the sad mom is able to have a healthy baby of her own.

The experience I come from (thank gods) doesn't involve any miscarriages, but years of infertility problems. I was so jealous of women who were getting pregnant and having babies that it made me bitter about them--and I didn't want to be! Honest! But I'd find myself scowling at them and thinking, "Why not me?" It ended a few friendships when I just couldn't keep my delicate emotions in check. I hated shopping malls where there were strollers everywhere. I hated babies. I hated kids. I hated moms. I hated everyone who was rubbing it in my face what I DESPERATELY wanted and couldn't have while THEY got to have it. Even complain about it!

I can only imagine the larger pain of a miscarriage. Her grief must be overwhelming, and she will always look at this other baby/kid and know that her baby should've been at that age doing those things. It will be a difficult mourning process for years and years.

Now, if she's a very strong woman, she will be able to see some light and happiness in the situation, but even the strongest woman will need a lot of time. And during this time, the friendship will probably suffer a lot.

How do I know for sure?
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Because my personal example is that when I finally, finally did get pregnant, a friend of mine who was desperate to conceive also got pregnant about 5 months after I did. Over 3 months later she had a miscarriage. After her miscarriage I gave her space, and she took it. She understandably didn't come to my baby shower (who could blame her?), and she stopped returning calls or emails.

How long did it take her to really make contact again?

2 years and 1 month. That's when she wrote me three weeks ago to let me know she is pregnant again and everything is healthy and well several months into her pregnancy. She can now talk to me again. And my arms are open to her. But for a while I wasn't so sure she was ever coming back, and she's the sweetest most loving person you could ever know. She just couldn't deal.

Best of luck to you, whichever side of the fence you're on!