Ethics aside, inform them directly of how this may negatively affect you and them.
You do not wish to send them the answers to your test because it may be traced back to you, causing you both to fail the course. No excuse of theirs will sway you.
Back to ethics, if there is rampant cheating, simply informing the school brass of it and dusting your hands is probably not sufficient and likely lead to the failure of legitimate students.
If rampant cheating goes on, courses often evolve cryptic examinations which are only solvable via dubiously-ethical methods, such as
- testing advanced topics not covered in the course (everyone appears to understand them because they are wrote copying well done on a great course, Professor)
- must have a copy of last year's test (see above bullet point)
- textbook has wrong answers which the professor treats as canon, and no amount of arguing will sway them
The existence of such a system, while it at first seems tragic (and is), is really another learning opportunity provided by Universities.
- people cheat, some cheat until they lose, and often they lose very hard
- sometimes a system can only be defeated by discussing the subject amongst your entire undergraduate class
- some courses teach nothing, but provide a wide swathe of peers to learn from
Consider working with a neutral member of the college to review the problems you've noticed once the semester is over, such as pieces of technology revealing the exam answers. Ideally this will also help retroactively correct the grades of students who did poorly in subjects because they were not part of team cheat (it has been discovered by review that course X had an unfair exam which covered subjects beyond the provided material..).
Justice is desperately important, but it comes in many flavors.