From which segment?
As far as I'm aware no airport in the UK does pre-clearance. DUB does, I'm pretty sure that's south of the border and so technically that's not actually the UK.
YYZ, YYC and YUL should all do it. Assuming your friend is on a UK passport they'll need a eTA for Canada as well as the US ESTA too. And they'll be doing a Canadian customs form regardless of how long they spend in Canada.