Just because she is getting married doesnt mean your relashenship will have to stop. it most liekly wont be the same as before,for she will be a married woman,but she will still want and need friends. Dont let her feel bad about dating or marriage because you will miss her,tell her you are os excited for her nexr stage and look forward to being able to help her,or encourage when or if she will need,dont make her feel bad about it.
start wtih the mindset now,as you ahve.that its not forever but just differnt.you will be friends,just she will have a ultimate best friend as well when she gets married