One faculty in our department just gave a talk on cheating. What you
described is definitely cheating with evidence. She reported to the
associate dean and the student got zero in that test. She always looks for
similar exam answers. That is one way she catches cheating students.
Sometimes, students copy answers that aren't even for their exams (due to AB
types of exams), clear evidence of cheating. Sadly, a lot of cheating
students are from China.
If you do not do anything, you could still get in trouble. Other students
might report you to the dean, as it is not fair to other students if the
cheating students get good grade. Other students might have seen it during
exams.