Right now, you can only open comments if you've logged enough progress on the buddy read book, however, there are cases where even approximate page numbers do not apply at all (in my case, a short story collection with the stories in a different order than in my buddy's book). To be able to see the comment my buddy left, I had to text them and ask them what page they were on and add that progress for my book.
This is why I'd like to suggest a way to open a locked comment regardless of whether you have logged any progress at all!