weed problem?

i hate to start another Dr.Phil thread but my girlfriend of almost 2 years wants to start smoking weed.......i used to smoke a shitload but havent done it in 4 years and shes only tried it twice. wtf should i do.stand my ground and say absolutly not.....and possibly be left out of a uncontroled situation.(her accidentally buying weed with crack in it)or cheif up. i mean....i used to puff on the bong all night long. i was the grand tokemaster....the red eyed cheeto muncher....the 30mph chillin like bob dylanator........but that was the past...but some of the worst memories happened...past girlfriends using me for weed......friends i thought were friends were only friends cause i was a smoker.......help me..please..