Friend has PND, but so do i - how can i help without exhausting myself?

my friend has 3 children and with her thrid (about 8 months ago) she's got post natal depression. im really worried about her, she's lost 4 stone and sys she ate 4 quality street the other day but felt so guilty she starved herself the next day. she had a stressful pregnancy and emergency c-sec, so had to see her little boy fighting in an incubator. she is very anxious about his development and health, which i realise are both signs. she gets very run down and has been in and out of hospital all year with varoius illnesses. her partner, although helps as much as poss, has to work awway a lot and she's on her own at least 1 or 2 nights a week. her 2 youngest were born a year apart and i think her BF is putting pressure on for a girl, as her oldest isnt his. if she has another baby she was told she's prob die though so im hoping she doesnt give in! they are due to get married in sept but in all honesty, i dont think she's ready. we're 23 and her fiancee is 33. she is petrified of being on her own though. i dont know what to do to help, she's quite a private person. i try to see her at least once a week, but ive also got PND and have done for the best (or worst!) part of a year now. im starting to recover but its a long slow process and i dont want to ruin what ive achieved so far by exhausting myself for someone else. i realsier that sounds selfish but ive been very ill adn i think i owe my daughter some good times together now. i want to call my firends health visitor or someone to tell them im worried but i dont want to go over her. ive suggested a very nice doctor to her, she was a star with me, but what else can i do? our friends are all at uni or working away so there's not many of our shared close friends around. she has a bad relationship with her mum, but is close to her dad although i dont know him myself.