Context is so important for what many would consider safe or not. We live in a small village, I am comfortable that my DD11 is safe walking back from an activity at 10pm once a week. Its a 2-3 min walk and I know what time it finishes and when she'll be home, and it means we don't need to get her toddler out at that time if one of us is working late.
However, if I lived in London, absolutely no chance. And the biggest red flag, OP didn't know her daughter was out. As she's said many times, if anything had happened, no-one would have known for 12-18 hours or so, as there was no information passed about her being out.
So it was irresponsible of the parent at the sleepover, she was in charge of the kids. The walk itself isn't necessarily a problem, not OPs preference and we don't know where it was for how safe it may be (or not). But the lack of communication meant that OP didn't know she was on her way, and the other parent didn't know that she had arrived home safely.
Parental preferences aside, it is important to take responsibility when we are caring for each others kids at playdates, sleepovers etc and if we aren't prepared to, don't have the kids over!