Thank you everyone.
We got its radiator cleaned and radiator cap changed. It worked.
The radiator was choked (as per the mechanic - don't they always say the same no matter what?).
Anyway, the cousin drove it for about 10 minutes in 2, and the guage didn't move a whisker. Earlier it started rising as soon as the RPM meter hit 5k.
Now, about the coolant - which one is the best? And does it have to be diluted? Or is it better to put it in concentrated form?