I was recently in a position where I was able to see the timesheet of a co-worker(private information, and I should not have seen it). Our standard work day is 7.5 hours with flexible overtime. They said they worked 9.25 hours but I witnessed them only working 7 hours including lunch. This person has been with the company a lot longer than I have and has a higher position.
Is it appropriate for me to report this? Are their risks to me if I do not?
No, unless your duties include signing timesheets/monitoring hours.
My advice is to stay out of it. Consider the following points:
Perhaps they worked from home?
Are you absolutely 100% sure that you know exactly what is going on, and are prepared for the impact of your accusation? Mistakenly accusing people of inflating time sheets will surely damage your relationship/career prospects.
Perhaps they worked overtime before and were not allowed to book it for whatever reason?
This happened to me personally - we had an artificial 'cap' on hours we were allowed to work, so management asked us to book our overtime at a later month.
It is simply not your job to monitor hours worked.
Once you start doing this, you are effectively stepping on your manager's toes. The dangerous implication is that they are not doing their job. The dangerous assumption is that they don't already know about this person cutting hours. Maybe this person inflates timesheets, and the manager is aware of it, but the person is so so productive that they are letting it slide. You don't have the information, or the authority to act on this matter.
What are you trying to achieve by this anyway?
There is absolutely no rational upside in 'reporting' your coworker.
One thing to note: if you feel that this employee is not pulling their weight, then you can approach your manager and tell her/him that. If you believe this employee hasn't been there when they needed to be, you can tell your manager that. As it stands, what you perceive to be a problem has no bearing on your work, and thus it should be none of your concern.
It is appropriate if you have been told by your manager to monitor your coworker's working hours. If you have not been told by your manager to do that, it's not appropriate. You run the risk of looking like you yourself are not doing any work. ("If you have time to log your coworker's hours, you need more work")
It is up to the manager to look for and detect these things. Snitching over petty things you are not completely privy to won't build a lot of political capital.