My dearest H-with-the-M,
Relax, take a breath, and do NOT call 911. Every good party always has that one guest who doesn’t know their tolerance, didn’t toke and wait, or ate the whole plate of cookies. The good news is, no one’s gonna die. Reassure them their green-out will be over soon, and make them comfortable. (Comfort depends on the intimacy of your relationship; a co-worker might want to be left alone in a dark room with a cold washcloth, while your little sister might insist on more personal attention.) Offer them (non-alcoholic) liquids, and a snack like a banana or granola bar. And give it some time.
For a more detailed answer, check out our deeper dive into overdose tips and remedies.