Rat my colleague out, or not?

I’m working voluntarily for/at a foundation that helps (ex-) addicts, (former) homeless people, and people with psychiatric problems.
All people that come and/or work there get a small allowance, about 4 euros per part of the day.
We fill in when we arrive, and when we leave.
This recording serves as proof to pay us.
Now recently I noticed that a colleague of mine (who works one morning per week, and who I know for ten years) is filling in hours whike he’s not been there(those hours), (thus acquiring extra euros per week).
Note that most, if not all, of the clients are very bad off, financially.
The money might come from a big money mountain (I don’t know how financing and budgets work), but I feel as if it is stealing food from their mouths.

What would you do in my case?