As a stuttering son of 16, I would ask her what she wants mostly. Does she want to do like reading exercises? Breathing exercises? Just speaking normally at the dinner table about her day? Keep it fun tho. Also ask if she comes to you or if she wants you to be the one that insists. And just study up on what stuttering is. Which kind of stutter does she have? Is it a hybrid? What is the source? In which situations does it get worse/better? There is a lot to it. I have had speech therapy for a couple of years so if you have any questions feel free to ask.