my friend lost his and can't afford a new fido phone.

IF your star tac is really CDMA then you have no chance.
If its a startac 3000 , the analog unit,
then  the only way it could be activated on fido is if you have a spare GSM
phone and roam with its analog service. But you need to be on a Month to
Month plan and it will cost you I beleive $0.20/min and that's charged
seperately on your monthly statement, and does not use you included minutes.

Overall its obviously not worth switching. You might as well stick with
Telus. And the phone needs to be CDMA.

Alternatively,
Buy a used unlocked GSM phone or a used Fido GSM phone and activate it, you
can find lots on ebay that sell from 15~20 $

And just a pointer, if you can't afford buying the phone, chances are...you
can't afford to pay for the service.

Hope this helps somehow.
DK