hmmmm, I wouldn't mind my own business to be honest! Yes people's parenting choices are their own, but you must not put a child forward facing until they are a minimum of 9kg/20lbs AND can sit up, unaided for a minimum of 30 minutes. So if the baby does not meet both of these requirements they are breaking the law as well as putting their child at risk. (not that the police care of course)
Also, how on earth would you feel if they had an accident and you hadn't at least tried to tell them? I'm a child seat fitter/advisor and I make absolutely sure every single person I serve is aware of ERF and the benefits before they choose a front facing seat, and I don't even know the children!
I think you should say something, but I think you should be very careful how you say it, obviously not wanting to offend or make them feel stupid.
UK law is actually changing next year to R44.05 which will require babies to legally remain rearfacing to 15months minimum. I think you should tell them that you are slightly concerned and print off advice from the british medical journal, rearfacing.co.uk and this from britax Your also welcome to use info off my website www.childseatsafetyplace.co.uk
Stress to them that your really not wanting to poke your nose in, and your not trying to convert them to ERF and it's absolutely their choice, but your just a little worried their breaking the law (as well as his neck if they crash) and that he'd be safer spending a while yet in his group 0+.
What front facing seat have they got out of interest?