“Single mum”

There's a difference between single parents and lone parents. Lone parents are completely alone. Single parents may share custody with their children's father.

They’re a mum who is single - single mum.

What gets me is the solo parent thing. The person in your example is not a solo parent.

Some people even use ‘solo parent’ when their partner is just away on business or whatever. My husband being away for a week doesn’t mean I’m a solo parent like my friend who has been alone since pregnancy and her child’s father is nowhere to be seen (not even financially).

I had a 50:50 arrangement with ex. The years I spent single I very much considered myself to be a single mum. I worked full time and had to support a mortgage, bills and my share of childcare costs on my own. If my car broke down or I needed to replace my boiler I didn’t have anyone else’s income or practical support.

Yabu.
Single parents are parents who have split and live alone, even if it is 50/50.

A lone parent has no contact with the other parent and does it all 24/7.

Was literally about to comment this!

There are single mums - those who are not married who have children, whether full time, 50/50, or whatever.
There are lone parents, who have no partner at all, whether because they are not at all part of the child's life, are dead, or only existed as a donor.
I think most people who use or hear 'single parent' understand the meaning as above. 'Lone parent' is sometimes used instead, but would be inaccurate. I think the same applies to the more recent 'solo parent', thought that seems to be used more in a casual temporary way, as in 'I'm solo parenting this weekend because DH is away'.

This comes up so much, you could just search and find the threads about it. It'll be everyone pitted against each other in a race to the bottom, as per.

It's nuanced. I have my kids 50/50 but in terms of my financial position, relationship position etc etc I'm very much single. And I'm a parent.

I don't think it's to do with being single, because if they were dating someone who wasn't the other parent I think single mum would still be reasonable to use.

OP I think this is where the distinction between single parent and lone parent comes in. When I read/hear lone parent I assume no involvement at all from the other parent. Single parent may have involvement to varying degrees. I'd say a lone parent is a type of single parent, so "single mum" encompasses everything from no involvement at all from the father, to a fully involved co parenting situation.

Because they are single and a Mum.

My ex sees our children every other weekend, has no input into their care/schooling/general wellbeing day to day. I work full time to keep a roof over our heads. I certainly consider myself a single Mum.

the ex couple agreed she will work fewer hours and take on more childcare and he pays mortgage, childcare and her maintenance

Yes, when you're not dealing with an arsehole... the majority of single mums need to work FT or multiple jobs to make ends meet.

They’re single and a mum. Or they may be dating but no longer with their child’s other parent. It’s not inaccurate even with a 50/50 care arrangement.

Lone parent is often used where the other parent isn’t involved at all.