I have a friend who is a chronic complainer. She wasn't always like this. I have tried talking to her about it, but she just blames me (& everyone else around her) for everything.
I have read a lot of advice that says the best trick to handling chronic complainers is to validate what they are saying. Th problem is she complains about things that don't affect her, for example:

She complained once about a couple she saw. They were just a regular married couple out for a nice evening together, they did not say or do anything to her. But she complained, saying she would hate to be the wife. I asked why. She informed me that you could "just tell" he was the kind of guy that would want his wife to make sure the kids were always perfect, she would have to always be perfectly turned out, etc. (it went on for awhile) In other words, she was judging someone for absolutely no reason. A lot of her complaints are similar to this.

Is it really constructive to validate in situations like this? Does it not just encourage the complainer to complain even more about things that in no way affect their happiness?
How do I validate something (like this) that is not a valid complaint?