Edit: about the meds... Ive stopped taking mine before, against docs orders (Im on mood stabilisers, the odd tranquiliser and something that puts motivation levels up), which in hindsight was a really bad idea. My mood will go right up, through the roof, and thats when I decide not to take em anymore. Shortly after, my mood crashes straight down - because of not taking meds - and thats when I do stupid stuff to myself. Then I start taking the meds again, and its another 2 weeks before my mood improves.
How long was it between when he stopped taking his meds and the party? Do you know the name of what he was on and how much he was taking each day? Seems to me like he went on a bit of a downer and tried to solve his problems by getting as mashed as he could that night

Edit #2: the girl, go for her, but be careful. Was she only saying that because she saw Robbie acting like an arse and was embarassed? Just be sure that she wants you and only you, because if she still has feelings for Robbie then things could get complicated. Hope it all works out well for ya though