How can i tell my friend she needs to see a dentist?

Maybe try flattery: Tell your friend about all of her awesome qualities that might not be noticed because of the distraction of her dental condition...Either appearace or breath for example. Maybe share how much you like your dentist (i hope you do) and how gentle and relaxed he/she is. Your friend will appreciate it when she finally gets the treatment she might need.

Tell her you: Are interested in her overall well-being. One's mouth is the basis for life -- we use it to eat, drink, smile, talk, kiss, communicate. Sadly it is often discounted as not being important. Big mistake -- till reality finally hits.

Refer her: Refer her to your dentist. Describe how great the office is and how comfortable you feel. If she says she has a dentist then tell her that you care about her and that is the reason you are bringing up the fact that she needs to see a dentist. Tell her that you want her smile to be as beautiful as she is! this will not be an easy conversation but you are a great friend to care about her.