I think she's simply got into the habit of peppering her language with lots of swear words and she probably doesn't even realise she's doing it - or how she sounds to other people. I think you just have to bite the bullet and tell her how you feel. Hopefully it'll make her think about what she's saying.
Tell her how beautiful she is and what a shame you think it is that every time she opens her mouth a torrent of foul language comes out. I bet she'd be mortified if you thought she had a potty mouth and I'm sure it'll make her think twice before saying the F-word in front of your nan!