This person stuck to her task and you see improvements and then a jump at 9 months especially.
She may have had other musical skills however, to ease the learning process her on this instrument but that does not take away from the achievement.
Getting away from the screechy first few months must have been the hardest, as there is a limit to how many repetitions of a simple tunes an adult can take, (I suspect).
I am not going to state the obvious and why I posted it here, other than this is typical successful adult learning. You stick to it ... you get it.