IIRC the only actual evidence is that Colonel Henry Bouquet wrote a letter proposing the idea. His commander, Amherst, verbally approved the idea but whether it actually went ahead or not is hypothetical. There's no evidence one way or another. Oddly, the Indians may actually have brought the smallpox epidemic on themselves. In 1757 at the siege of Fort William Henry (in present-day upstate New York), Indians allied with the French ignored the terms of a surrender worked out between the British and the French, broke into the garrison hospital and killed and scalped the patients, some of whom were suffering from smallpox. The blankets and clothing the Indians looted from the patients in the hospital and corpses in the cemetery, carried back to their villages, reportedly touched off a smallpox epidemic.
There's another problem here as well. Prior to vaccination as we understand it, exposure to attenuated smallpox was seen as a viable means of preventing the worst ravages of the disease. The Chinese used the ground-up scabs of smallpox ulcers which the patients inhaled. Reportedly it worked quite well with only one recipient in 30 dying and the rest surviving both the vaccination and any smallpox epidemics. Other cultures used exposure to cowpox (works very well). The British and French used blankets from smallpox victims that had been stored for long periods and then given to children. This also worked, most of the time.
There is a final codicil to the situation. Indians have severely degraded immune systems. It's bad now and it was much worse back then. American Indians were and remain notoriously vulnerable to contagious diseases. Some scientists have theorized that the Asians who migrated over the Bering land bridge millennia ago were exposed to such intense cold that the diseased among them died on the way. Isolation from Eurasia and Africa insulated North and South America from such contagious killers as bubonic and pneumonic plague, smallpox and tuberculosis. Accounts of the era speak of Indians who joined settler communities died of disease with remarkable rapidity. One might suspect they were assisted to do so but there are parallel accounts of settlers who married Indian women only to see their much-loved wives expiring as well. One account speaks of a trader who nursed his desperately-ill Indian wife and, when she died, shot himself so he could return to her. Archaeologists who have examined natural or manmade Indian mummies have discovered that prior to the settler's arrival, Indians suffered from cancer, arthritis and, rarely, tooth decay but not much else.
There is not doubt that the great smallpox epidemic centered in the Mississippi Valley was a humanitarian catastrophe. Huge numbers of Indians died, and along with them went the hope that Western Indians could thrive as farmers. Because the worst-hit tribes were agricultural, the Indians themselves came to view farming as a death trip. That was a cultural catastrophe, a heart-rending tragedy—but it was not premeditated genocide.