This probably won't work for a couple of reasons.
I assume when you say hank on main, you're actually referring to a mainsail
that currently has slides or slugs on it.
If that's the case and the sail was a conventional main with battens, you would need to recut the leech to add some hollow. If you're not a sailmaker
experienced with using a lofting batten, you probably should turn down the project
at this point.
Assuming that's not the case and it's a batten-less main with leech hollow, then we need to be concerned with shape.
Most frequent complaint with in-mast furling sails
is that once the sail starts to lose shape it won't furl properly. If someone is getting rid of a sail, there's a better than even chance that the sail is not in the best of condition and that the sail has a "belly" to it.
mains are inexpensive to build. We are building a genoa
and staysail for a Roberts 44 ketch
that we met while cruising. The boat has an in-mast furling main. The main has some UV damage to the clew area. We haven't checked to see the shape, but if the shape is gone, we'll probably build a new main since a recut and repairs
could represent a significant percentage of the cost of a new main.
Furling mains are relatively inexpensive. There's no batten pockets or reefs
to deal with. Unlike a furling headsail,, we only need a small suncover for the clew.
Most likely your friend would be better served with a new sail.