Me thinks he probably doesn't have canvas plyers either.
Gotta put my educator hat on here and step up on the soapbox. Start of rant........
Canvas pliers should not be used to stretch a painting that is already on canvas.
A stretcher bar that is fixed with gussets in the corners is not a stretcher bar. It is a strainer bar and is inappropriate for restretching a painting that is already on canvas.
Using canvas pliers and a fixed strainer means the only way to tension the canvas is to pull it with the pliers over the fixed bar dimension. It is very easy to pull the canvas out from under the gesso and paint on the surface - but you may not see the damage (stress cracks) for a period of time. This is over tensioning.
Sure, it can be done (using canvas pliers and fixed bars) - but works on paper can also be hinged with duct tape. Doesn't mean it is proper or the right way to do it.
The proper way is with a properly size adjustable bar, attaching the canvas while it is lying face down by initially tensioning by hand, and then doing a final tensioning the canvas after it is affixed to the bars with hand tension. The keys or adjusting mechanism should then be fixed (so the upper keys do not dislodge and drop behind the bar) or a mechanical bar using draw tite fasteners should have shims placed in the open miter.
Regardless of what you use, a rigid backer, preferably 4 ply cotton backed with fluted polypropylene (Coroplast) should be affixed to the stretcher.
End of rant.