I have bought a mobile through a friend in the US. What is the best way to get it to India without getting into trouble while departure from US and on arrival to India (customs)?
Just carry it with you. You are allowed to carry personal items, such as phones, with you from one country to another.
If it is still new in the box, it might raise flags that you're importing it, rather than carrying it as a personal phone, in which case you may be required to pay import duties.