Don't buy merlot if she knows about wine. If she doesn't know about wine, then buy merlot. I am not trying to come across as an asshat or a wine snob. If she does know about wine, then merlot is not interesting and she would probably like to see you take a risk on a different red. The fact that she told you red or riesling probably means she is not overly into wine, so you can buy a merlot. It is pretty hard to eff up merlot grapes, so anything in the $8-$12/bottle range will work just fine.