I don't think he could play for the country of his Wife's birth.
I think it's all about someone above you in your family tree coming from that countr, thus making you part, of the country.
For example if his granfather was India, he'd been some proportion Indian, therefore he could qualify to play for India.
It doesn't work the same if he just marries an India woman, because he is just English and South African by decent.
However, if he married an India women, and they had children, the child would be part India, therefore could play for India, after serving a qualification period, if he had lived abroad.