The Terbite above referring to "the lounge" at the Fairmont Royal York presumably means the RY's Library Bar. On the RY's 2nd floor, The Station might be an alternative. It is actually Toronto's smallest bar, and sometimes empty in the afternoon, so that might be a good spot, depending on your timing. I am uncertain whether it's open in te evening.
Staying with the Hotel Bar theme, you can try the King Edward. Very nice and usually not too busy. There also used to be a bar around there called the metropolitan.
If she's wearing business attire, why can't you select from the hundreds of bars around town? And if she's wearing clothing that makes it look like she's in the business, is any place sufficiently discreet?