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Did it look like the suspect was based nearby? Might have an arrangement with the "victim" whereby the first to wake finds breakfast for both then wakes the other.
Not your job to work it out though, just report it. Homeless people who steal from other homeless people probably won't refrain from other acts of theft, so pursuing this case might help reduce the overall crime levels in that area.
Moral question. On my way to work today I saw 1 homeless man take money from the collection of another homeless man who was asleep. Should I have said something?