I couldn't do it.
Please don't tell a single mom that you couldn't do it. You could if you had to, which is what they are doing too. It sounds like a nice thing to say, but the reality is that it makes it seem like they have a choice. Often that's not the case. They do it because they don't have a choice. This is what life handed to them and they are doing the best they can.
What other things do you think are not okay to say to a single/divorced mom?