visitor in transit visa - please help

[win99]

HI

My friend is traveling from US to India via london heathrow. He got Indian passport

1. His 1st leg of the flight is USA to London heathrow.
2. His 2nd leg of the flight is from London heathrow to India.

2nd leg flight is 3 days after the first flight and it seems he can take "visitor in transit visa however while filling the application it is clearly mentioned " you should not use the visitor in transit visa to visit family and friends in uk" which is very confusing. My plan is if he can able to meet friends with "visitor in transit" visa he can change his flight to travel within 48 hours

> Is he allowed to apply visitor in transit visa if he is going to stay above 48 hours (he need to stay in uk for 3 days for his next flight)
> Is it possible to apply the transit visa when he arrive at Heathrow airport .?
> I am in UK so do i need to submit any documentation to support my friend "visitor in transit " application if he is applying from India.?

It would be gratefull if someone can answer these questions. Thanks in advance

[win99]

Can someone please help me here. Thank you