If I started cutting into my friend’s stereo or a rental car they may not like that.

Tsk, tsk... have they no faith in you?

But yeah, one might be in a situation where a hardware mod, while possible, isn't an alternative - and for a temporary setup those solutions are great. In cars, cassette decks seems (at least around here) to start getting scarce - specially in new cars - so the FM version might be the most universally appliable.

But I see no obvious reason not to try the hack if he has any confidence at all in his hardware skills.

/Michael