I bought my girlfriend a Trek 7100WSD almost 3 years ago when we first started dating. She rode the spare MTB I had at the time (frame cracked and I put the components on a new frame for my brother) a few times with me on the fine gravel Canal path near her house, and liked it, but wanted a more comfy bike. She loved the ride of the 7100, so I bought it for her ($320). We go on rides all summer, up to about 25 miles and she likes the upright position the most. Specialized Globe series would be worth looking at, as well. Stick with 700x35 tires, they roll fast but at the same time are wide enough for gravel or even hardpack dirt.
And by the way, she makes a great cycling partner. No shes no where near as fast as I like to go, but that means I get to work on spinning and keeping my heart rate up for a good cardio
We average about 10mph on the canal path, whereas I average around 13 last time I took my own trip on my mtb, but I also wore myself out good. The first few rides shes ALWAYS the one that keeps going and going... It is also a great way to spend time together, that and kayaking (and we have a TANDEM kayak, known as a "divorce boat"!!! we have become quite proficient at paddling as a team...) are our favorite warm weather activities together
When shes got the energy, she will race a little here and there on the trail, too...