Crochet is simply hooks, yarn & a pattern, most of us, especially an avid crocheter likely has a full set of hooks that is her favorite (and a back-up set that are not her favorite). Patterns can be really subjective.
Well, what we need most is yarn and yarn someone else pays for is the best
Yarn someone gives us and doesn't expect us to make something for them is awesome.
However yarn is also really subjective, some people love acrylics, others plant fibers, others wools & animal fibers. Not to mention color & weight. An how much yarn you need is dictated by the pattern.
Have your son snoop at her yarn labels, if she uses Red Heart, Bernat, Lion, and several others, usually acrylics or blends a craft store gift certificate would be great, find the one with the best yarn selection.
If she has higher end yarns, wools, silks, etc (the brand names are extensive & vary by region) a gift certificate to a local yarn store would be better, they have better quality yarns and a gift certificate would allow her to buy the yarn she secretly desires but hasn't been able to justify buying.
Good luck!
