Since your friend got converted into Hinduism before marriage the marriage could have been solemnised under provisions of Hindu marriage act 

for valid Hindu marriage it is necessary that both parties are Hi dus 

If she was converted to Christianity before marriage but got married under Hindu marriage act, then that marriage is invalid.

Besides being a foreigner, her marriage should have been registered under special marriage act hence the marriage is not valid on that count also.

Her marriage is very much valid. But she can get the marriage dissolved by Indian Court without coming to India. She can also get the marriage dissolved in U.S. 

Marriage is valid for all purpose till not dissolved by competent court.

As your friend was married in India under the HM Act to a person converted to Hinduism from Christianity at the time of marriage, it is perfectly valid and legal. Her becoming a US citizen later shall not alter the situation.

- If this marriage was solemnized when she was not attained the majority , then this marriage can be declared as null and void by the court. 

- Further, if at the time of conversion her consent was not there and under his influence she adopt the Hindu religion then this marriage having no value in the eye of law. 

Under Indian law, a marriage solemnized under the Hindu Marriage Act is a valid marriage regardless of the nationality of the parties to the marriage. Therefore, if your friend was married under the Hindu Marriage Act, her marriage would be considered valid in India.

Both should be Hindu and Indian citizens. Her marriage will not be valid if not registered in india. She should have opted for special marriage Act in above facts