Can't speak for OP's girlfriend, but depending on your city's recycling system, oily tuna cans in the recycling can cause a problem. If your city is singlestre

Can't speak for OP's girlfriend, but depending on your city's recycling system, oily tuna cans in the recycling can cause a problem. If your city is singlestream, then your oily can will co-mingle with paper products, which can then contaminate the paper products - our systems cannot recycle oily paper, and relies on sorting to remove them from the recycling stream at processing sites. Also more personally, depending on your recycling/garbage storage conditions, keeping around food residue may cause unpleasant odours or critters.