Can anyone tell me where the Prison in Chennai is?
The one where a forigner on drugs charges would be held?

Near the central railway station .

There are 2 railway stations in Chennai(previously known as Madras)
one is Egmore where you get trains that take you to places inside
tamilnadu(the state of which chennai is the capital)

the other is known as 'Central' - central railway station where trains
leave to other states in India

If you are going to India by flight you can take a
cab(taxi/autorikshaw) to central station and everyone knows the
location

There is a small bridge from Central station road towards Mount road
where the prison is.

let me know if you need any more information.

My advice would be to contact the Indian Embassy / US Consulate in
Chennai before trying to meet the person. It is not mandatory that
they keep all prisoners in one place, especially foreigners. So, get
the help of US Consulate, in case you are an american citizen, or
Indian Embassy to trace the prison where he/she is held.