Grandmother is medicating my son without his consent.

I feel angry and worried. My Mom shared with me today that she has been medicating my college aged son for the last few days in the coffee she makes him in the morning without his consent. My son recently moved back home after he graduated college this past summer with his Masters. He has been going through a difficult time, he did not want to move back home (he has been living with roommates for the last 4 years), his job ended, lease was up at apartment, bad break-up, generally feeling unsure of what direction to take in life and he wants to take a break from all responsibilities. He is only 24 years old and already has his Masters. He does seem a little lost and met with a counselor twice in the last month. My mom feels he just needs medication, she shared this with him and he said no, not at this time. He wants to try to figure life out for now without drugs. Respectfully I will honor his wants, however I plan on keeping a close eye on him. Then my mom goes and puts prescription depression medication in his coffee. I told her under no circumstances is she allowed to do that to him or anyone, she made a face and rolled her eyes. I feel as his mother I need to tell him and as her daughter, I should not rat her out to her grandson, I am also angry with her for thinking that is okay. What do I do?