At my workplace, the company has trusted us with full access to its Cloud infrastructure service account (think of Azure, AWS, etc). The belief is for developers to have access to what they need fast for development. Recently, I happened to stumble upon something and discover that a colleague on my team is using the company's Cloud service account for purposes related to his personal project. I've found out that he uses the resources in the company's Cloud service account during and after working hours for his own project.
I asked that colleague about what he was doing with those resources in the Cloud service account. He vaguely replied that he was doing something which sounded something related to the project we work on but it was clearly a lie because the code he is running isn't doing what he said. When I looked at the code he ran, he appeared to be running something related to his personal project which I could see was pulled from his public Github account.
The cost of these Cloud services are based on our usage. The cost he has incurred on the account is not significant enough yet to alarm anyone as our monthly bills for Cloud services are huge. So nobody is likely going to notice that.
I'm having the urge to tell this to my manager because I feel that this is ethically wrong and he shouldn't be doing so. However, I don't know how my manager will respond. From my experience working with this colleague, I find that he has issues with being honest and he might be able to lie his way out. The last thing I want is for my manager to think I'm being petty over it or start judging that I was too "free" at work to check on my colleague or even think that I should support my colleague doing his personal project. If my manager is going to confront him, things can get awkward between us too.
Rationally, the cons of telling my manager outweigh the pros. After all, it's not my money. I could pretend I don't know anything about it. It feels like I saw somebody stealing and I kept quiet about it, and I don't feel good about it because this is unethical.
Is it better for me to turn a blind eye or should I inform my manager?