Thunderchild, this is very considerate. May I ask, how old is this young lady? You speak of "late bloomer" as if she's not quite so young.
If I were you I would not be over-personal, to say how she's grown in confidence is more like the sort of comment you would make at a personal appraisal - I would say more something about how much "her contribution to the Rotarians has increased so much in recent months and she is a valued and outgoing member of the club, with all her talents coming to the fore" - or something like that. That doesn't acknowledge that she was not outgoing before, but does allow you to say how much you appreciated what she does for the club. How does that sound?