What happens if you don't pay the credit card or personal loan amount, and then leave the country?

Your friend will be shut out of the credit market in the United States for a few years and that's it. (For people with deeper ties to the United States, this is seen as a horrible option too because it means they can't get a mortgage on a house and have known bad credit for basically a decade, or more.)

But this is treated as a default and the banks and collection agencies have no way reliable to collect outside of the country, and most likely won't attempt it. What they will do is try to sell the debt for a very low amount, the person that buys the debt for cheap may attempt to actually collect the money as they stand to make a HUUUGE profit (for example, they might buy the debt claim for $500 and still try to collect over $150,000 + interest). So it is possible he will be followed by debt bounty hunters, but most likely nobody will buy that debt when they do their basic research and find out that you - I mean your friend - went to India.

Your friend will regain access to the credit market in the United States faster if they officially file for bankruptcy. Some people don't want to do this either because the bankruptcy court records are public, even if it stops having a negative effect on your access to the credit markets in just a few short years, public perception can bring it up decades later, as seen with top US public officials and Presidents.

Outside of the US, a US debt matters very little - financially and reputation wise - and a lot of people do skip town after amassing large debts. Typically this just raises the interest rates on unsecured credit, analogous to how insurance pool premiums rise for everyone else due to some people abusing the system. The banks just mitigate risk on their lending bets and keep making money.

This answer specifically is not about the moral or principle opinions of these actions, and only the actual consequences.