And even if it was possible to activate it by making a phone call while roaming, according to the direct link that inquisitor posted, all three roaming partners of Wind in Canada allow incoming but not outgoing calls/texts.
So I suppose that the only option you have left with, is to activate it when you will be back in Greece. Or if you are in a hurry and you want to take advantage of the 1.5GB/mo
free data offer, you have to send it back to your friend in Greece for the activation of the card and the offer.