I would also vouch for coinbase and localbitcoins since it is easy to buy bitcoins there in many countries. I also suggest coinhako, it is the fastest way to buy btc in Singapore IMO but you won't be anonymous if btc transaction volume is large(since it will require identity verification), fees are also low. I also recommend unocoin for your Indian friend, it is the best exchange to buy into locally in India(but it is regulated by Indian government). For your American friend, he should just use coinbase.