I think that it doesn't matter what it was that he took. He went to someone else's desk and removed an entire tin of biscuits and, presumably, ate the lot.
Theft, greed, just plain selfish! Even if he thought they were "office biscuits" does that make it ok for him to take the tin?
It is the principle. The biscuits are irrelevant. He took something that did not belong to him. Theft is theft, whether it's a tin of biscuits or a wad of cash.
tbh, I suspect that there is indeed, as has been said, more to this than is reported. Clearly he is a selfish arse who thinks it's ok to make off with a tin of biscuits from someone else's desk. I bet it's not the first time he's done something like this. Such selfishness tends to be a trait, not a one off.
Perhaps she was pig sick of him thinking "what's yours is mine and what's mine's me own"
I once lived in a house with several teenagers . one of them had a habit of taking food from my cupboard. One day she snaffled a tin of spag bol (yes, I know, but I was young and disgusting ) and I hit the roof. She offered to replace the tin but I said I didn't care about the tin! Try as I might, I could not get it through to her that the tin itself didn't matter, it was the fact she had no respect for my belongings. I bet this is the same.