you can't and shouldn't even try. trying to change others or make them do what we "think" they should do is a very common mistake of a lot of people when dealing with others. We can only advice, support and listen to others. So with that if you've told her how you think she should behave with a therapist you've done all you can. You can do nothing to force her to comply and if you could, would that really be a good thing?