When I want to delete a user, it's not sufficient to just delete issues, you must also delete comments, but how do you find which tickets a user's comments appear in? With the issues, you get a link to show all the issues so you can then do a bulk delete. Not so with just comments.
Why do you delete a user? Why not simply set the user inactive, remove the user from all groups as well.
To answer your question you can find issues which have been commented by a user with scriptrunners jql function
issuefunction in commented("by <User>")
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