I have been sewing canvas with a Singer Heavy Duty 4423. It's probably twice the weight of good jeans denim, and the machine has no trouble going through two or three layers. It does seem a little fussy about stitch length and tension though. No trouble punching the needle through, but the thread (heavy duty stuff, on the limit of what will slip through the eye of the biggest Jeans needle I can find) will "birds nest" underneath if the settings aren't just right. I don't think it's a problem with the machine as it sews perfectly on lighter fabrics, just some issue with the feed dogs actually moving that much material evenly. Get it right, and it's fine, get it wrong and you're in a world of jammed needles and knotted up thread.
If I did more of this type of work, then perhaps a walking foot would be a good investment?
As Mr Twingo says, the machine itself isn't everyone's cup of tea, but I like it and it does everything I can reasonably ask of a domestic machine, and probably quite a bit more.
Also, bear in mind that "canvas" can be anything from fairly thin fabric that any machine would handle, to heavy sailcloth that would need a massive industrial machine to work with.
Do you know what kind of work your neighbor is actually planning on doing?
I'm reluctant to suggest anything I have not tried myself, but a friend has a Toyota Super Jeans machine, which she says is "like a charging rhino, nothing stops it". About a hundred quid cheaper than the Singer too. Might be worth a look?
As for leather, again it depends very much on the material.
Back in the days when I was making knives, I would wet-form the sheaths from 3mm (1/8") veg tanned hide.
Sewing through three layers of that stuff, again as Mr Twingo says, is best done by hand.
I'd punch each stitch-hole through with an awl, and saddle stitch with two needles, crossing through the holes.
The only machines capable of handling that kind of work were the old-fashioned cobblers ones, and even the vintage machines were way out of my price range.
Nowadays, for lighter, garment-weight split hide, or PU leather, the Singer can plough through with no drama at all.