Going to second the recommendation to file a report, not least because it is a felony in most states, like USNA 19 DAD wrote. I know this seems like a craptastic thing to do to a friend, but there's another reason I urge you to do this. Starting with the extensive "secret" clearance questionnaire you're going to have to fill out before R-day, and continuing throughout your Army/DoD/government/possibly NGO careers, every time you apply for a security clearance, you will have to disclose this. And if you think that "secret" questionnaire is extensive, just wait until you get to top-secret questionnaire and process. When (not if, when) you have to explain this, you want to be able to do so with reference to a police report ("Smithville Police Department #2016-1234567899") rather than "Wait...you got into USMA, knew that drug use was a future separable offense, and you got stoned anyway?" "Well, you see, it was a friend of mine, and I didn't know the brownie was spiked." <pause> "Yeeaahhh...riiiiight..." <marks checkbox for further intensive followup> You have to be really selfish here.
But as for the short-term effects: THC is eliminated very rapidly from the body: a few days for non- or very-light users, and up to a month for frequent or heavy users. So the THC and other compounds will be long eliminated from your body by the end of June. Also: this is not your fault. You are not "in trouble," here. Your appointment isn't in jeopardy because you haven't signed the contract yet and you're not yet the property of Uncle Sam.