Paralel: I believe that's something to do with the Mac drive heads having higher magnetic coercivity or some such - that is, they exert a stronger force on the low-level magnetic field of the diskette than a PC drive does.
I could have that the wrong way round.

But yeah, differing magnetic exertion.

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Of necessity, floppy drives are matched to the characteristics of the media with which they are designed to work. And 720K floppy disks are the same as 800K Mac disks, so there is no difference in "magnetic exertion" or coercivity (which is a property of the media, not of the heads). High-density (1.4MB) floppies are also the same for Macs and PCs, but differ a bit from their lower-density ("double-density") ancestors.

That might also explain why when my friend tried to replace the floppy in his old mac with a non-mac part it couldn't read anything.

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Because Mac floppy drives use a completely different interface from that used by PC floppy drives, the non-functioning is to be expected. Indeed, there is a great risk of damaging the Mac (and drive). Cross-species transplants should be undertaken only with extreme care, and only after some research, or you risk a release of magic smoke.