I cannot answer your question, but I can point out a couple of things.
First: your question reminds me of my mother's concern when I was
living in NYC, and she was seeing riots in Harlem on TV in California.
They were as distant from me as they were from her.
Delhi is a very big city, not all the streets are flooded. There are
always health risks, regardless of flooding, but your son is probably
not going to be taking his girlfriend to wade in the flooded areas.
The flooding should not make Delhi less safe for Westerners. They may
feel very upset by the plight of those suffering, maybe feeling
cultural shock at the sight of poverty unknown in the West, but the
Indians are accustomed to it, to the so apparent contrast of rich and
poor; they may beg aggressively, but not attack Westerners just
because they are better off.
Your son and his girlfriend will probably be sticking to the tourist
sights, and the folks there know that tourists are their bread and
I hope that assuages your misgivings a bit. No one can give you
absolute assurance. Tourists can have problems in American and
European cities because they are less cautious and more likely to
wander into the wrong area.
Yes, I have been to India. The last time, a driver came back and
returned my camera.
Try to relax. "No news is good news."