There is a very little chance that US embassy will reject your visa if you have done your undergrad here in US and if you are enrolled in graduate program. As long as you had decent grades in undergrad, maintained your immigration status and as long as you are enrolled in a good graduate school with financial assistantship there is very little chance that they will reject your visa..I went to nepal to renew my visa after undergrad and i got it easily. I showed them all the documents like before. So I guess there has to be some reason why your friend;s visa was rejected...without knowing that reason..i don;t think we can give you proper advice...anyways one option is to ask an international student advisor to write a letter to US embassy..may be it will help...goodluck