My Dad ran a greenhouse for years, and is an avid gardener. His advice to me is that you should avoid watering leaves if you water in the evening, because the water doesn't evaporate and sits to potentially cause rot. In the morning, it's not as important.
However, it depends on the type of plant - some types of plants it's really important for (like tomatoes and young cucumbers/other melons), and others it doesn't really matter at all (carrots).
The rapid evaporation point that PFHC made is an interesting one I had not heard of before.
Bottom line, from my understanding, is that your SO is more right than you :)