Let's think of all the things that could have happened by you telling a drunk girl that you can count in order of danger/likelihood of happening.
1.Her saying outloud, "Did you know this guy's a genius. He can count cards." You've now blown your cover and everyone will be paying attention to you
2. Her getting very pissed at you if she trusted you enough to raise her bets in a high count and she loses. Ploppies don't know crap about variance usually. Your cover is blown and you have an angry drunk girl on your hands
3. She actually wins and starts to really like you as a result. She goes back to the room with you and you contract an STD. Depending on the STD, this could easily be #1
4. Same as #3 but you could be robbed.
5. She actually has the presence of mind not to say anything and pays attention to you, but the synchronous raising of the bets by you two leads to a sharp pit critter catching you. Your cover is blown.
6. She wins with you and the pit critters, dealers and players are totally oblivious to what's going on. You have some nice company for a little while
7. She's a smoking hottie that's single and you score!
I give #7 a 1% chance of happening
Best bet is not to do it!!