Report someone to HR?

Do not do this; as a broader recommendation if you aspire to have a career in finance (and make it through) is to not meddle in other people’s business, especially if it serves to no affect to you.
Another point: you should know that HR in finance =/= HR in other corporate fields. People always get an impression that HR runs the show… that is not so in finance. For example, if you report to HR, consider these two scenarios, equally likely:

1. They LOL about it (they very likely can not GAF) and gossip amongst each other at breakfast. Inevitably, this will float to bankers (frequently HR have friends who are bankers / even work on the same floor as a team) that you snitched. Once you begin your time at the firm, you’ll begin to wonder why the bankers are side-eying you or very short with you. 
2. Let’s say HR does GAF. They discuss internally and try to revoke this kids offer- BUT that senior banker/MD/Group head really likes the kid and personally recruited him. Said MD/group head demands to know why and who posed the accusation. In this situation, you should expect the questioned HR person to meekly say “[your name].” Do not misunderstand- when the person who pulls in double to triple digit millions to the firm asks a specific question to HR, they will answer. Given this, you’ll be in bad faith with MD/Group head and in that case, well, you won’t be going to that firm full time.

All of these things are perfectly avoidable by minding your damn business and not trying to play God.