I just say, well don't complain to me unless you are going to do something about it, and if you aren't going to do something about it, then stop complaining.
I dunno, when I was crazy sleep deprived with my first son sleep was ALL I could think & talk about so I complained a fair bit. I just wanted a sympathetic ear though as I had no intention of 'doing something about it'. I know some mums in my mother's group didn't like hearing from me because they felt like you but I think women should be able to talk honestly about hard the lack of sleep can be without being expected to try and 'fix it' and sleep train their child.