As a stutterer for many many years I would say do not give any advice or tips, that probably could backfire big time. But I think you could talk to her and say like you have seen that she is having harder to talk in some situations and if there is something you can do for her or if there is something she would help with training to feel better talking in those situations. There is a lot you can do for more fluency and feeling more at ease speaking in situations where it is more difficult. But... Do not give tips or advice - let her decide if or what she would like help with. Stuttering is very individual and one person who stutters does not have the same experiences as another. It could be so that she feels that despite having it harder in some situations she is fine with it and does not want any help or she would like help. Anyway.. ask her if you want but drop it if she doesn't want any help.