Cheating during a test
A good question!

There is not much you can do now. You cannot prove anything and can not even name the cheating person. If you think that explaining the situation can help the invilgator to make cheating less likely the next time, you should write them an anonymous email.

I've heard that there are some universities with so-called "honor codes" that tell you to report this - if this is the case for you, do absolutely report it (anonymously).