@used123: In post #5 above you indicated that you will speak with your son which suggests that you had not yet done so. Speaking with your son about your concerns should have been your first step, in my opinion.
Based on your description of your son’s best friend and his pattern of dishonesty, I do think that it was poor judgment to allow him to use your computer. Seems more likely that your son would allow him to use his computer since they were best friends.
Without proof that this young man did all of the things that you assert, I think that it is unwise for you to jeopardize his standing as a student and his reputation at school.
OP: My response in this thread was based upon your request for advice.