Old_G....wrong....it is the male prerogative to give the "green light"
Tradition... honour...good manners.....dare I say it....a "gentleman"..if the woman wants to avoid the label "slapper2 then she needs to be patient OR woo her man.
Why not just start a conversation saying you have heard he is leaving and what is he going to do, then go on to say you must buy him a drink before he goes. If he turns you down he is probably not interested and you can easily shrug it off and wish him luck. if he accepts see how it goes. What have you got to lose.