100% approved by our army of gals and our field report earns us a few bucks.
Advice
Pick the right size. You can choose from a few different sizes, from a personal mat to a two-person mat. The medium sized mats are the best for most gals because they are larger than a standard sized beach towel, but they're not quite big enough for two people. A mid-sized mat will give her a lot of real estate to relax on, but will still feel like it's all for her, since it's designed for only one person to use at a time. If you get her the two-person size, then she'll feel like it's a gift for the both of you. Don't do that. You want to let her know that this is a special gift that's just for her.
Don't borrow her mat. Understand that this mat will be exclusively for her. The moment that you begin sharing it is the moment that it ceases being her gift, and becomes just another tool for the family to use as they please. Go get your own mat if you need one, or buy a separate giant mat for when you want to take her on a picnic...but leave her personal mat alone.