1) Single moms are my heroes.
While I completely understand and appreciate the sentiment behind this, it's also extremely hard to believe that any woman that has a child and is not married is your hero. I mean, seriously, think about what that means. Saying something like this actually makes you come across as disingenuous.
2) I could never do what you do.
I have often wondered if I could ever be married. I am not saying that I wouldn't do it or that the desire has never been there but if I think about all that goes into being married (that I am aware of), I really don't know if I could do it. However, it has never even once crossed my mind to say that to another woman. Never. I think the fact that so many women respond to single mothers in this way, speaks to the fact that there is still a certain stigma that goes with being a single mom. That it's still a pretty foreign concept to many when, in fact, about 12 million families in 2016 were reported to be single parent families, over 80% being run by single mothers. This isn't some rare situation that takes supernatural abilities. And, furthermore, it could happen to any mom.
3) I hope you're getting child support.
I firmly believe that both biological parents need to be responsible for the financial needs of their children. I also think it would be lovely if no mother ever had to play debt collector or had to serve papers to get financial help from a child's biological father. Unfortunately, that is not always the case and it's often easier said than done to those things. Right or wrong, for many women it is a journey to get to the place of filing for child support. Let her take that journey. If she seeks advice, by all means, give it. Otherwise, unless she is asking you to support her child, there is really no need to bring it up.
4) Are you able to support your child financially?
Let's be honest, most people aren't asking this because they want to cut the single mom a check; they're asking because they want to know if the government is. While I personally am not in the situation to need cash aid or food stamps, many single moms are. Yep, even the "working" ones. There is a massive difference between the average income of a single mom and a two parent household, yet the expenses are the same. Don't be that person that makes someone else feel bad or ashamed about their situation.
5) That must be so hard.
Every mom has hard stuff. Married. Single. Engaged. It doesn't matter what your relationship status is; there's always tough stuff. I know I always feel a little awkward when people say this to me. To me, this is just my life and I just do what I gotta do.
6) Where's the dad?
This seems like such an innocent question but it's often quite loaded and leads down a very personal road. If a mom hasn't volunteered this information, it's probably best not to ask.
7) You must be so lonely.
I'm sorry but did you miss the "mom" part in single mom? A single mom is almost definitely not lonely.
8) It must be nice to have a break from your kid.
While every mom likes a break once in awhile (and should take one as part of her self care) this is a different situation. I personally haven't gone through this, as I have my son 100% of the time, but I've watched many parents close to me go through this and going from having your kids with you full-time to kid-sharing is not a vacation. I think after awhile you adjust but it's still rough and not exactly "nice". (I am personally not looking forward to the day I have to "share".)
So, what should you say?
The same things you say to married moms! Talk about their jobs and Netflix or being a mom in general. There's no reason to pry into personal things like what happened in the past with the father (oh and PLEASE do not ever refer to my child's dad as the "sperm donor" or my "baby daddy".) or where her money comes from.
Most single moms did not picture themselves in this situation .Even those that use donors probably didn't have childhood dreams of being a mom in this way. It's certainly not how I pictured life as a mom but, for me, this is my normal. It may not be to you and that's OK. You may have questions and that's OK, too, but please when you are talking to a single mom remember that she is a person who may have traveled a pretty rough road to be where she is. Let her decide if and when to tell you that story.