If an app ruined part of your site I'm not so sure I'd be that generous with the rating. Any decent app dev (or anyone on the shopify expert listings for that matter) should weclome the good with the bad. Pressuring someone to remove a review is just sneaky.
I'd suggest reaching out to shopify and making them aware of the tactic that dev is using. There's potentially other similar apps that deserve more credit but since they haven't cheated their way to a high rating not as many people notice them.
Sidenote: What had broken on the site btw? I see your site has some weirdness in the footer where the search bar is.
