Apart from the obvious risk of the greenhouse being damaged if the mature fruits drop through the glass; the tree is probably blocking a lot of light from the greenhouse too. It would be best to discuss this with your neighbour to warn him of the problem. Try to gain his permission to remove the pears overhanging his greenhouse. Perhaps you could invite him to harvest them himself and let him keep the fruits.
If it is possible to prune out the offending branches this winter then this would be advisable to prevent the problem recurring next year. However if there are too many branches overhanging the greenhouse then you may need to consider removing it. It would be a great shame as your Pear is obviously doing well, but it is really not worth falling out with your neighbour over a fruit tree. You can always replant another pear tree somewhere more appropriate in your garden to replace it.
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