A friend showed me an article saying that there's evidence that a piece of Halley's comet may have struck the earth about 1,500 years ago. Is anyone here aware of such evidence?
Little pieces of Halley's comet hit the Earth every spring. It's called the Eta Aquarid meteor shower, but I think you are thinking of something bigger.
Dallas Abbot's paper notes that at the 533-540 point on the GISP2 ice core there are Sn-rich particles, Ni-rich particles and cosmic spherules. These she identifies as evidence of a minor cometary impact. The resolution of the GISP2 core is sufficient to place this impact in the late spring, consistent with the Eta Aquarid meteor shower, which is known to be produced by Halleys comet.
Dallas notes that an increase in cometary dust in the atmosphere could partly explain the extreme weather events of 535-536, in combination with a minor volcanic eruption. Although there are other, competing hypotheses, and this is not considered to be settled science.