There isn't currently unfortunately, there's not a great way to query github for this data.
You could view the git[hub] history for a directory and follow it back to the pull requests (github has a "history"
link when browsing a directory, clicking a commit link on that page will take you to a page that includes a link back to the source PR).
Alternatively you could search PRs to the repo for _any_ reviews by that user and then see manually identify relevant PRs.
Typically for the specific case of people filing a PR for becoming an approver, they link to evidence themselves including PRs that they know they've reviewed and any other way they've contributed to the relevant directory for the target OWNERS file.