Your wife needs to feel supported in her decision to wait until six months, and not have to justify it to you or anyone else. It is up to her to decide what she thinks is best for her baby and for you to respect that decision.
I chose not to wait, but that was the decision I made for myself and my babies, and I feel strongly that it should always be the mother's personal and individual choice.
I followed my own instincts with both my DC (now 15 and 17), as I felt that they were ready for food. My DH was supportive, despite being more cautious than me by nature. He would have been inclined to follow government /NHS guidance if it was left to him to decide, but rightly felt that the decision was mine to make, and not his.
I was the only one in my group of friends to start weaning early, and resisted a lot of pressure from them, as they all thought I should follow the 'official' advice (mainly, I suspect, to validate their own decision to wait). My health visitor was surprisingly supportive though, and told me that guidance is just guidance and not a rule, and the six months recommendation was fairly recent (this was in 2005). She said it was taken from the World Health Organisation recommendation that babies were exclusively breast fed for 6 months, as this was the safest and most hygienic advice for mothers in third world countries where keeping equipment sterile was difficult and water for formula and baby rice could be unsafe for babies.
I didn't do baby rice at any stage, but started my DC with fruit and vegetable purées at 17 weeks and they were both fully weaned and eating everything by the time my friends with babies the same age were introducing food to theirs at 6 months. DS and DD are both extremely fit and healthy teenagers. They have always been great (and unfussy) eaters and have had no ill effects whatsoever from starting food at 17 weeks.
Every mum should be able to make her own safe and informed choices about what is right for her baby at every stage of their development, without feeling pressure from anyone. I mix-fed both of my DC from birth and I had so many people telling me 'oh no, you can't do that, it will confuse baby / reduce your milk supply etc.'. But I ignored them and did it anyway. It worked really well for me and my DC, took the pressure off establishing breastfeeding and made it very easy to wean them off breast milk when I was ready to do that after four months.