I have no idea what to talk about with a certain friend...?

You have to be urself!! Ur worrying soo much about what shes going to think about u that u wont just be you. In order to be best friends u need to be 100% YOU. If she cant accept that.. Then it wasnt meant to workout. When some one is sad or upset, mostly all they want is someone to LISTEN to them. If u dont kno what to say, thats ok! Just show ur concerned and that u have sympathy & u care. (: