That’s not how you prune a tree like this and your gardener should know that. You basically paid somebody to ruin your tree. It doesn’t look like your die back is from that hack job thou but I would replace that tree anyways since it will not ever look right with all the support branches cut. Also taking off that much at once will probably stress your tree out and send out a bunch of water sprouts which grow from lateral buds and are much weaker then normal branches as well as causing you way more work to get your tree back to a healthy state. Here’s a article on water sprouts if you wanna know more about them

https://www.thriftyfun.com/Removing-Tree-Suckers-And-Water-Sprouts-1.html