My standard reply to such questions:
Make up a "bouquet" of samples for her. Arrange them in a basket or pretty little glass object d'art (floral glass frogs work well), and you'll have a real visual gift. By choosing a variety of scents, you're most likely to locate at least one that will thrill her and become a staple in her stable of scents. She'll have the fun of trying different fragrances over an extended number of days, and you'll avoid the risk of buying a big bottle of something she might not care for. If she loves one more than another, then buy her the bottle for Valentine's Day.
Here's a list of the many places that sell samples: