We will explore three halachic issues, starting with lashon hara. If one sees Reuven wronging Shimon monetarily, he may take steps to protect Shimon’s rights at the expense of defaming Reuven, provided that seven conditions are met.[2] Since your intentions were good, if you were sure of what you saw, the main condition left was the need to rebuke the culprits gently before telling others in order to give them the opportunity to rectify the matter without embarrassment.[3] If that had proven ineffective, the laws of lashon hara would not have prevented you from alerting the driver to claim the money that his company deserved.
The next question is whether you were required to take such steps. Accordingly, we must discuss the two potential gains that would warrant action: helping the young men morally and helping the company financially.
In general, there is a mitzva from the Torah to rebuke someone who commits a sin.[4] This mitzva is strongly related to the responsibility of afrushei mei’isura (to distance a counterpart from sin). Included in this mitzva is not only the instruction to rebuke in a way that will cause deep remorse leading to the sinner’s decision to turn over a new leaf, but also in such a way that will discourage the imminent, or rectify the recent, perpetration of an individual sin. In your case, the young men were in the midst of making use of the bus illegally, as they apparently had no intention to pay. Thus, rebuke was ostensibly called for. However, if one thinks about it, we have numerous opportunities to rebuke people on buses and in other public places for a variety of their actions regarding matters between man and HaShem and between man and his fellow man. For better or for worse, we rarely do so. Our general working assumption is that since our generation is proficient at neither end of rebuke – giving or receiving – we have more to lose than to gain. One could argue that a clear man-against-man sin, such as sneaking onto a bus, is one that everyone would admit is inexcusable, and the rebuke would therefore work. However, many people, likely including the young men you saw, are able to rationalize such activity or simply do not care that it is wrong.
The final issue is hashavat aveida (returning something lost). Although the most well-known discussions of hashavat aveida refer to physical objects, the mitzva also includes a variety of actions that are required in order to prevent loss to others. For example, if one sees that his friend’s property is in danger of being damaged, he must protect it.[5] Furthermore, it appears that according to the consensus of poskim, one is required to take steps to help his friend receive the money that he is owed. Thus, for example, in addition to the specific mitzva to testify on a friend’s behalf, several poskim say that the mitzva of hashavat aveida also includes going so far as testifying to enable someone to collect money due to him.[6] No matter how we classify the aveira of getting on a bus without paying,[7] the bus company has legitimate interests, and your action presumably could have helped them.
Taking everything into account, however, it is likely that you were still not required to do anything. One is not required to put himself in a situation of significant embarrassment in order to carry out hashavat aveida.[8] Since asking the offender to pay and/or going to the bus driver to “snitch” would likely have caused a very upsetting experience, you were likely exempt from doing so.