dear Simon,
I understand your concern but also know how difficult is it to give 'un-asked-for' advice.
Parents have to see the problem themselves, you cannot change that, but being a friend you could give them some information.
Below a few samples of articles on the subject.
[Presence of genu valgum in obese children: cause ... [An Pediatr (Barc). 2003] - PubMed result abstract in english, article in spanish
http://www.idosi.org/wjss/1(1)08WJSS/5.pdf article about older girls
ISPUB - Study of Knee Angle Development in Healthy Children aged 3-16 years in Ahwaz, IRAN article on study of knee angle development (iran)
Hope this helps you a little.
kind regards
Esther