Another here on the 'let her pick it out' bandwagon. When I purchased my current machine (about 15 yrs ago now), DH was with me. If left to him, I would have ended up with a more expensive machine with capabilities I had no interest in using. As it was, I got the machine I wanted that does the things I wanted it to do. Haven't regretted that decision one iota. It is a Viking Husqvarna that has been an absolute dream.
Perhaps put a picture of a sewing machine in a card/box and wrap that for birthday or Christmas - occasion of choice? That way she knows of your intent of the gift but gives her the joy/need to select her own machine.