Unfortunately you will hear similar to this many times. The reality is that until you are a single parent you are unlikely to realise or understand the full impact of what is is like, especially the lonliness of it. I think of it a bit like when I became a parent. Until it happened I didn't really understand the enormity of it. I know now that until you become a parent and have that responsibility day in and day out you have no real clue what it is like.
So it is with being a single parent. You think you know what it is like, but until you are there and living it day in and day out you really don't have any clue as to what the reality is.
So your friend probably doesn't understand, or at the very least doesn't understand how it makes you feel when she says something like that.
I do find myself pointing this out to people now as it can be wearing hearing the 'i'm almost a single parent' from people who really aren't anything close to the single parent experience. My own sister said this once as her dh works shifts and she felt like she was doing it all on her own and was 'practically a single parent...' Yet her dh was still home a few hours later having picked up the shopping they needed, was the one taking her ds to his karate lesson the next day, would be having the kids so she could go out for a couple of hours with a friend in the week, and had texted her several times while I had been visiting.
Your friend probably is exhaused after a week or her life being different to usual. And unthinkingly she thought she was experiencing what you do. Maybe she even really believes she is experiencing life as a single parent and that you'd understand how she felt and that what she is feeling is how your life is. She won't be the last who says this to you but whether you choose to point out to her the differences or not is up to you.
I've been doing this for 16 years now and, personally I would now say something, whereas in the early days I would probably have bit my tongue and kept it inside.
I would probably point out to her that being a single parent is relentless, exhausting and there is never any time to recouperate or recover. That I don't have anyone to call on when I'm ill or need support, or who would pick up the pieces should anything happen to me, or be on the end of a phone if I need someone to talk too. And if nothing else most DP are at least contributing to the household income rather than like me being soley reponsible for bringing in every penny in this house. So yes it must be hard for her doing that for a week but how lucky she is that is is only for a week, unlike single parents such as yourself who do it constantly with no-one coming to help them at the end of the week.... Phrase it as appropriate for your situation/friend and how you actually feel.
If she is a good friend she will understand and hopefully not say it again. If not then maybe she's not such a good friend as you thought after all.