About the same as they can teach them to drive a car.
ie it works for a select few but for others.......
I know quite a few instructors who have ended up in long term relationships with thier students. The common factor is that most of the students never complete thier PPL.
As for being friends with the student it all depends on the instructor and student.
Its a bit like other forms of work. Some folk become friends with customers and socialise with them others do not. Some solders are quite happy going for a pint with there NCO's but most are not. Its not wrong but shouldn't be considered normal.