Already here @LandSharksAnonymous
Okay, he's a Maremma. I'm Pyreneans so not dissimilar. They are both among the more healthy and sound giant breeds structurally - much better than the real heavyweights. I'm also glad to see that his crate is open. That's good to hear.
They are very, very sensitive dogs but you will already know that so I'm kind of saying it for other people's benefit. They are 100% committed to family, but everyone else can go to hell. More or less.
I'd say it's typical adolescent male LGD and he just doesn't like her in his space any more. Although friendly as pups, they become a lot more aloof as they get older - some more so than others. They also become more territorial during adolescence. They're calm at home but if something gives them the hump they'll let you know.
Really, it's anyone's guess as to exactly what happened but if she's been pushed for time and hauled him bodily out of the crate that would be enough to really bother him. If he's like mine, he won't react to you (gently) hauling him around but if anyone else does it he will not be happy. These dogs are total control freaks, they don't like being rushed or pressured, they do things in their own time. It does not make them the most co-operative group of dogs.
I agree with @tumblingdowntherabbithole that as he matures, group walks may not suit him so well so play that one by ear. Once they start to have an issue with a particular dog being in their space it can be game over. Our previous PMD boy was placid until he spotted the Malamute from across the road, then all hell broke loose. But that was territorial so tours might still be okay on neutral ground obviously.
We've had 8 PMD (4m, 4f) and been lucky enough to be able to walk them ourselves, but if I was in your position I honestly think I'd look for a different dog walker.
Again, I'm stating the obvious I know, but I'd be very surprised if he forgave her for whatever has offended him.
I really hope you can find someone he gets along with, so he can enjoy his walks again.
It's a wonderful breed ❤ I love all the LGD but the big white ones are the best!