Photinia problem

Hi all,
I am new to this but have a curious problem with a Photinia. A 'weed' tree appeared and grew so fast, very close to the Photinia. It was blocking the light to my front room, was not particularly attractive, and we decided it was going to cause problems structurally to the building if allowed to continue and my neighbor sawed it at the base.
The Photinia a few feet next to it has since wilted.
Would anyone here know whether this would be caused by the proximity or some sort of symbiotic thing between the trees that has been broken and so stressing the Photinia?

I am not sure whether this is the right place to ask, but to anyone reading this, help would be gratefully received.