His first mistake is being friends with a single mother (especially one with such young kids). Even ones who aren't deliberately trying to use you end up doing so, because they are cesspools of need. Raising two kids is hard enough with two parents, let alone one. Maybe he was friends with her before she had kids, I don't know, but regardless, that's just a dangerous situation.

This is true even if, up to now, she managed to not ask him for much. Single mothers have very little reserve left, be it energy, mental capacity, money, support, etc. if you're around them long enough, they will have a meltdown or breakdown of some sort when a stressor arrives (she loses her job, the kids act up, baby daddy drama, whatever) that she can't handle alone, and you'll feel obligated to help.

This is what I suspect happened to the OP. He might not have been helping her much before. Maybe she managed to keep her single mother needs under control in their friend group. But it's clear she was hitting a breaking point when she's saying she needs a break and a vacation. That should have been a warning sign. People go on vacations to take a break from a lot of things. 9 times out of 10 a single mother is looking for a break from her kids.

Finally a few notes about vacationing with young kids, for those among us who are childless :) it's been said that vacationing with young kids is just taking care of them in a different place, one without all your shit. It can sometimes feel like that. You're left wondering if the sights and sounds of the destination is worth the fact that, even after stuffing multiple suitcases full of their stuff, it's still not as easy to take care of the kids as if you were home. A two bedroom suite is nice but it's probably still not as big as her house and certainly doesn't have everything setup for their daily routine like their house would be. This means taking care of them is usually harder than being at home.

Secondly, families with nannies often travel with their nannies, precisely because otherwise it doesn't feel like a vacation, just taking care of the kids at a different place. Here's what it costs: you must pay all of her costs, including a separate hotel room (unless you're airbnb'ing a large house where she can have her own room), plus give her set times when she can be alone and not taking care of the kids (usually set times each day, plus like a full day off if it's a week long vacation), plus pay her her regular salary for that time (sometimes more like 1.5 since she's away from her own family for this time). That's the baseline, and sometimes you end up paying even more, like giving her some spending money (per diem, even after you've covered all her meals and lodging), and of course, bringing her on any excursions, day trips, etc that you may have planned.

It adds up fast (both the inconveniences of not being home, and the expenses of childcare) and at least once during the trip, you'll wonder why you didn't just do a staycation at home :)

If he knew all of this beforehand, he would have gone running. I'm glad he eventually wised up and ran.

(That said I have to wonder. He's a single 26 yo guy. When I was his age, one of my goals in vacation was to hook up with girls, either locals or tourists letting their hair down. Even if this friend of his didn't have kids, why would you vacation with her, and have her cockblock you all week? Even when I was having sex with someone regularly, I usually preferred vacationing with guy friends as it was a lot more fun to go out and flirt with new women, or overall let loose, than spending the whole week in a hotel room with the same woman, who's expecting you to do couples stuff all week).