A friend or an acquaintance, that when booze is involved, proceeds to become your drinking enemy. This can be viewed in a positive light but extreme caution needs to be in place as the sole motivating factor for a Drinking Enemy is to take you down with them.
Greg, you are a great guy but you have made way too many drinking enemies in the past. Or,
Adam, you may be my drinking enemy, but will you be my Best Man?