Lara was a Croft. That alone meant she knew how to dress well when the time came - before her parents' death, she'd attended enough events and such with them to learn the dos and don'ts of pretty much every situation.
Of course, some of that went out the window when faced with Sam, but she wasn't about to admit that.
When she turned up, she couldn't help a smile. "I didn't keep you waiting too long, did I? I was just running a few checks on a few things..."