If only the tube were shorter than a hacksaw blade, he could put the blade into the tube, then mount it on the hacksaw frame, clamp and cut straight without any trouble.
How wide the desired kerf (slit)?
Here's another approach, that will cut clean blind slits:
Make a jig of course (Hi I'm Kobuchi). Screw two 2x4s together along their lengths to form a long "L" profile cradle for the tube. Secure the tube into this with clamps or screws through the oversized pipe's ends.
Now fashion a cutter. This could be a block of wood with the point of a long sheet metal screw (or drywall screw - both are hardened and will deeply scratch copper) poking through it, the block simply worked back and forth until the cut is complete.
A better cutter would be made by sawing half way through a wood block with an old worn out hacksaw blade, then snapping the blade (the broken ends will be sharp and obviously hard enough to cut even steel) and securing a blade shard in the block with one ordinary wood screw, as a setscrew. A point of hacksaw blade will scratch off a nice thread of copper at every pass.
This method will yield cuts only as straight as the 2x4 cradle. 2" or 3" angle steel would be an improvement, if you have it. The cutter needs a bearing surface, or you're just cutting freehand (yuck).
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