I mean a lot of women prefer quartz you know? and the eco drive is a good option considering you probably won't have to worry about battery change for like 10 years.
the thing is at under 400, you're not going to get a swiss mechanical diver new and there aren't that many small real divers (IMO 300 meters +) so nothing on the used market. So you are left with Japanese or Russian. Most of the Japanese divers are 41mm +.
There are some dive model Seiko 5s that are small but I don't think the depth rating is enough, maybe only 100m.
I guess you could try getting her a round Vostok Amphibia. Its rugged, 200m, mechanical, 39mm, and under $100. Finish is not good though and accuracy will be in the 20 to 30 sec per day range. If she is a Wes Anderson fan though, you can get her the one that Bill Murray wears as Steve Zissou in the movie.