Is it a good idea? Probably not.
Does it nonetheless happen all the time? Yes.
My father taught me to dive 3 years before I was first certified (and about 10 years before he first got certified). He started diving about 10 years before PADI came into existence, and that was how diving was taught back then. Experienced divers taught newbies.
Does that mean it is a good idea to do it now, when professional training exists? I think the question answers itself.
Does it nonetheless happen all the time? Yes.
My father taught me to dive 3 years before I was first certified (and about 10 years before he first got certified). He started diving about 10 years before PADI came into existence, and that was how diving was taught back then. Experienced divers taught newbies.
Does that mean it is a good idea to do it now, when professional training exists? I think the question answers itself.