Keep in mind that some folks are really, *really* good at complaining, pregnant or not. In fact, when they think about conversing, something wrong is always the first thing that comes to their minds - and the second thing, and the third, ad nauseam.
I'm not talking about mamas who are having a spell of morning sickness or a spell of the blues. Everybody goes through that, and everybody needs a friend for that. I'm talking about the chronically negative ones, women and men, who complain so much you wonder if they're expecting a reward for it.
If they're not pregnant, they're having trouble with the spouse, or the in-laws, or the neighbors, or the schools, or the house, or the town, or the political situation, or the weather. Chronic complainers are very skilled at what they do.
In fact, back when I found I was pregnant with our first child, I was so delighted I wanted to broadcast the news from the rooftops - until I listened to the other mothers I knew and their *nonstop* complaining. Then I wondered what awful mess I had gotten into! I was devastated. Then I decided to find other friends.
What I would do, I think, if I were in your shoes would depend upon how close I was to the friend. I might say, "What's the BEST thing that's happened to you today?" every time I saw her. If she asked why, I would tell her why; some people just need to realize what their mouths are doing. If she was offended or if I didn't know her that well, I guess I would just put up with it - or not.