I can't think of any points offhand where jetBlue's and Westjet's network overlap, so it would be difficult to feed passengers to one another.
A better alternative would be to permit jetBlue to purchase Westjet or another carrier and relaunch it as jetBlue Canada. This is currently forbidden under Canadian law, which drastically limits foreign ownership of airlines. But the crusade against foreign ownership has grown stale and dated in this era of free trade agreements, and some in the ruling Liberal party are in favour of legalising foreign ownership, along the same lines of what Australia did a few years ago.
And it's an idea whose time has come, too.