Hi there, we lived in Miami a few years ago, and traveled regularly in and out of the UK, so we know a bit about US immigration and can tell you now, you are probably on a hiding to nothing - US Immigration will DEFINATELY NOT be amused by your friends drug conviction and it is unlikely that you will be admitted into the country, unless you lie on your Green Card as you go into the US, and if they find out you lied then you will probably be sent on the next plane back to the UK. First offenders who are caught smoking cannabis etc in some US states get sentenced to LIFE imprisonment, they take drug offences so seriously over there! Good luck, and we hope you do get there and enjoy your vacation, cos all said and done, USA is a great country!