Mike, its pointless trying to empty it of rainwater if next door refuse to have theirs cleaned becuause the next rain storm they will be full again.
Explain to your customer and explain that the dirt and moss from next doors guttering could be washed back into theirs.
If you feel you do want to empty the water out to make you feel better and you don't have the vac? then simply remove the end cap and any water will flow out, not all of it will come out because the fall of the guttering will be flowing towards the down pipe.