I think your idea is really good, nonny2t! Hopefully that would make her realize she has no idea what she likes and who she is. At 23 years old... I would agree it's a sign that something is wrong.
Maybe it could be a good idea too to reasurre her as well. Tell her would still like her if her tastes were different, if you didn't like the same movies/clothes/music/etc. I think sometimes people do that to make sure they are accepted.