if its possible for him to change his flight details to delhi then he would have a lot more to do here....he could go to agra to see the taj mahal, go to 2-3 places in rajasthan or to see the ganges in varanasi....

while there are things to do near mumbai as well, i would pick delhi & the nearby cities over mumbai any day....

taking the train would take him a long time....if he is traveling for the first time & is not too street smart (as you stated) then i would suggest traveling in either the ac second or third class section....ask him not to get a general ticket....

$1,000 for a 10 day trip should be sufficient....