Transit Visa

Hi Guys,
one of my friends is planning to take a transit visa to UK. He is going to India from US and planning to visit UK, on the way back to US. The transit visa allowance is 48 hours it seems. What that 48 hours meant? The gap between the two flight times?
The visa requires biometrics to be given. Can he give the biometric s when he is in India or whether that has to be given in advance? Any sort of suggestion would be very helpful..

If he's applying for a transit visa, he needs to go e biometrics (which is needed before processing starts) after his application in India (I guess) tog get a visa.

Transit visa isn't supposed to be a way to visit the UK - it's just to facilitate travel. If he wants to visit the UK, why not go for a visitor visa?

Agreed, he needs a visiting visa not a transit visa. Transit visas allows you to enter a country and catch an outgoing/connecting flight from there, not to exit and visit/tourism.