Buddy, how does it really matter?
Look at it pragmatically (as a Parsi, I have common ground here .... with Hindu guys replacing Muslim ones for you).
The girl is lost to you anyways in terms of lineage.
Her progeny are never going to be Hindus.
Does it matter if she remains Hindu or converts? If I am not mistaken, except for a small group of Hindus somewhere in south India who practice matrilineal lineage, Hindus too have patrilineal lineage rules.
Cheers, Doc
Gauri's kids will question islam and will explore Hinduism. They will understand dharma and respect it. Down they line they will call themselves "atheists" (due to the strong brainwashing against Hinduism) like Javed Akhtar. The next gen will then find their way back into Hinduism.
Salman Khan's entire family has married Hindus. From his father who got his mother converted, (but she became a hindu again) to his brothers who married hindu women without conversion and his sister who married a hindu man.
THeir children will understand dharma and respect it too. They will continue to find Hinduism part of their identity.
The daughter of that CPIM leader has left dharma forever. That is the difference.