Red wine is typically 12-14% ABV, beer is typically 5% ABV. Anna is holding a large glass of wine (250ml), so a third of a standard (750ml) bottle. Anna's glass of wine is similar, maybe a bit stronger than a regular (500ml) bottle of beer.
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In their last visit to the pub, Anna drank three glasses of wine, so a whole bottle of wine, in let's guess under two hours while they were playing their game. That's enough to make most people drunk (not sex with Marvin drunk but certainly judgment impaired).
I don't think Kendra needs to spike Anna's drink for
something to happen with Marvin, just start a drinking game / dares, and move on to cocktails or shots. Maybe a badly timed message from or about David triggers Anna to get "revenge".
What do we think she does with Marvin? I don't think it will be anything too exciting. Maybe just more under the table play.