My friends an airhead and it's driving me crazy..?

She's not the brightest, but lately she has been so .. i can't even explain. It feels like I'm constantly thinking "GET WITH IT!" because she is acting like an airhead. For example, I am attending a funeral for the regards of a teacher, and everyone was aloud to go. I asked her if she wanted to go and she said "Well.. I don't know what time.. and I don't know what to wear.." so I told her "Well, it's at 3:00 P.M and wear something formal and black." She responded by saying "Would black skinny jeans work..? And my mom won't be home tomorrow.. so I'm not sure about the time.." And I told her that if it was to inconvenient she didn't have to go, in which she responded "I don't even know what that means..! :D " Sometimes I just want to rip my hair out.. she can be so much smarter.. but she acts like an airhead and it is so annoying. How can I be nice without getting to annoyed with her..?