For me, in order for it to make sense for Robin to be faking drinking a spiked drink, she would have to know it had been spiked. She will have to be shown in a flash back seeing it happen. Otherwise, she would be faking drinking a spiked drink that could be not spiked. There is also the question...
If Robin isn't faking it, wearing a wire, and or had someone watching her back, then this story just took another rediculous turn. The idea that she would have an open drink that could be spiked, while trying to catch men that spike drinks, makes zero sense.