tbh. the thing is that I don't believe she actually is buying the drugs for the girls at the club. If they all took drugs like Lily claims they do, then wouldn't they already have a dealer anyway? Cause keep in mind, Quinn is in financial trouble and trying to solve that problem by making all her drugs more expensive. Seemingly all the way up to doubling her usual prices. Her customers in the college are only allowing that because she happens to be their only dealer. So why would Lily buy massively overpriced drugs from Quinn, when her or one of the other girls could get them cheaper?
To me this mostly smelled like Lily wanted to check if Quinn can get her amounts that are too high to be considered "for personal use" in order to use that to fuck Quinn over later, no matter if Lily is working for Quinns rival or for the cops.