I wouldn't say that you or anyone else is in a disadvantage carrying a 9 millimeter. There are plenty of American soldiers in the graves from World War 1 and 2 from this round. If she shoots a 9mm well and feels comfortable with it, then she will find it will suit her well in most self-defense situations. All of the common semi auto rounds (9mm, .357 SIG, .40 S&W, and .45 ACP), with proper shot placement and a reliable modern JHP will do the job.
As for Taurus PT709 Slim, it is not rated for 9mm +p+ because there is no SAAMI rating for a +P+ cartridge. The Smith and Wesson M&P manual even warns firearm owners not to use it. If you choose to use this +P+ ammo, do so with caution.
That being said, Winchester Ranger T is an excellent type of JHP. I also recommend Federal HST, Speer Gold Dot, and Remington Golden Saber.
The only disadvantage a 9 millimeter poses is the risk of injury or death to an attacker.