A sample from this plant must be mass spectrographed in order to identify its chemical constituents. A toxicologist would be the one to handle these matters but it will cost you much money. Once you find out, if the plant was poisoned, what are you going to do since you did not see anyone do it. What I see in your photo is a dead plant and a big spiderweb which you can get tangled up in if you commence to make unsupported accusations. My advice to you is to throw the plant away along with the dispute and any bad thoughts you might have. Wecome to GOY:)

I think its unlikely that some sort of toxic substance has been poured over the plant and into the pot - I can see grass growing in there, along with a green leaved plant or weed to the left of the picture. If the plant had been sprayed with herbicide, it would have had to be particularly lethal to kill a woody plant - most herbicides will just knock the leaves out, leaving the woody stems largely intact, and if something really awful like Paradise had been used, then everything in that pot would be dead and the soil within it poisoned.

Its much more likely there is another cause for your plant's death, maybe root rot or something - whatever, I wouldn't reuse the same potting compost for another plant in case there was some fungal problem which caused its demise.

The usual holly is the common European holly, Ilex aquifolium. This is a large tree needing lots of water. I would not have thought it particularly suitable for pot culture and suspect that this one may have been left dry for too long. As Bamboo notes there are green weeds growing in the pot so poisoning is unlikely.