It shouldn't grow again. It should be his decision not his sister's. If it were me I would wait until I could get a referral from the GP and take the advice of the NHS consultant. If the consultant confirmed I had a cataract I would the n decide whether to go private or wait.
Is it noticeably affecting his vision? It must be 20 years since I was told I had a cataract starting in one eye and only now can I see the effect and it's still not bad enough to need it doing. I'd leave it.
I had both of my eyes done by the NHS. Cataract surgery involves removal of the affected lenses so no reoccurence can happen.
I'd ask the optician for a referral letter for your gp and arrange a consultation with the NHS clinics.
Both my eyes have been done with a 6 months gap between each operation, previous to this there was a year to wait, nhs doctor said the cateract had to be a certain size before he would operate,and he was the best in his field in Glasgow,my advice would be ,
Wait and get it done on the nhs.
It wouldn't grow again. It's just a question of whether he wants to live with it as it grows and get it removed for free later, or get rid of it now and have better eyesight for longer.
I had both mine done on the NHS, a few months apart. Both under GA, though I don't know if everyone offers the option. I was surprised at the difference it made, not just in sharpness of vision but the world changed colour. I had one blue eye and one yellow eye for a few months in between.