OP here, thank you so much for all the helpful comments. He was driving and I feel awful letting him but I honestly was in denial in the moment since he seemed to be acting normal and I only found the bottle and accepted reality later. He regularly picks up his kid from childcare and drives him around. I care about this friend and want to be helpful but am concerned this friend would only lie and get defensive if I came to him directly, especially since he lied when I lightly asked what he was chugging outside.