For your purposes I like the Beretta 92FS/92A1.
It's a very aesthetically attractive gun, making it more appealing to girls (I promise I'm not making this up as stupid as it sounds. I've taken girls shooting with friends who were bored with my friends' Glock and XD type guns but ooohed and aahhhed at my Beretta).
It's a very friendly gun. Low recoil, easy to handle.
It's a very reliable gun.
It has a heavy first trigger pull and a manual safety to ease her mind.
I'd grab a 92A1, a GunVault safe for it, a Streamlight TLR-1 to fix to the rail, and fill a few magazines with a quality load like Federal HST 147-grain. After all, if you only get one, go full-out.
Actually...if you really only get one...you might go look into a Sig Sauer P226 or HK P2000. Something that can last bulletproof for ages.
13 Pistols, 5 Rifles, 1 Shotgun, no time to shoot them