Here's a link to the Elections Canada application form for getting on the international register of electors list.
As well an application for Registration and special Ballot for Canadian electors living outside Canada can be obtained by email; firstname.lastname@example.org or toll free by telephone in Canada and US i-800-463-6868 in Mexico 001-800-514-6868
According to the form's instructions, an ex-pat's address for voting purposes could be their last home address in Canada, or the current address of any relative of themselves or their spouse or common-law partner, or even "a person with whom you would live if you lived in Canada"...which seems like it could stretch to cover any close friend.
Mobilizing this expatriate vote favours the opposition parties since it can avoid vote-splitting. A more detailed outline is available in this brief PDF: