Yes definitely valley tiles rule as they keep water on top of the roofing instead of into a gulley of some kind that can cause slates or tiles to slip or block up causing water to flow underneath and into the roof void.
Perhaps he didn't, assuming that is he and not she, know any better and has taken the job on without ever by the looks of it tiled a laced valley or ordinary tiled valley before.
Problem with you tube is that everyone incapable of any trade skills can say yes and think that you tube will save...
Yes you are correct, the battens are not lined up so you can't do the tiling without it leaking.
Fit a roofingwall valley if you get stuck. roofingwall.com