Just let your friend pull her hair out, ar no, cry herself out!
1) Firstly, remind your friend on the condition of her
relationship, the relationship may has its faults and just as the
other comments as suggested, take two hands to clap 2) Next, stay
by your friend's side and accompany her to do her daily tasks ie
grocery shopping, lounging at a common friend's place 3) A little
suggestion, choose a day out and enjoy fun of motor activity (swim,
yoga, trek) or mental activity (monopoly, scrabble, monodeal) dat
said, no use crying over spoilt milk, your friend will strive and
your friendship thrive!