But don't despair. According to Cancer Research UK, survival rates are improving all the time. In the 1970s only five out of 10 women survived the disease beyond five years: now the figure is eight out of 10.
So what can you do to help a woman with breast cancer?
Most of us feel a bit hopeless because we don't know what to say. We're scared of blurting out the wrong thing. But as Anne Rafferty from the charity Macmillan Cancer Support says, there are no magic words. 'Often it's not about what we say – it's just that we are there, and that we listen.' It can be a huge relief for someone who's just been diagnosed, or is living with cancer, to be able to talk, especially if she's frightened or anxious. There's brilliant advice on how to be a good listener on the Macmillan website . Sometimes, if she's telling you how she feels, non-verbal communication - a touch on the hand, or a hug - is the best response you can give.
Help with the kids
Offer to take or pick up from school or nursery, especially if your friend or relative has hospital appointments. Having her child regularly - say, every Tuesday after football - makes it easier for her to plan ahead. If she's in the middle of treatment and completely shattered, it might be a good idea to organise a rota of friends to cover the whole week. You can help best with pre-schoolers by organising days out to give her a break, or by offering childcare in her own home while she rests: toddlers sometimes need the reassurance of knowing that Mummy's just in the next room or asleep upstairs. The charity Breast Cancer Care suggests various books that you or she can read with the children that might help, including The Secret C – Straight Talking About Cancer by Julie A. Stokes (Winston's Wish, £4.99, for ages 7-10). You can also download a book for children under six called Mummy's Lump, by consultant child psychiatrist Gillian Forrest, from their website.
Do the practical stuff
There may be times during treatment when she's too unwell or too tired to cope with everyday jobs like shopping or cooking. Good nutritious food contributes to recovery, so take round home-cooked meals for the freezer or just keep her fridge stocked up with fresh vegetables. Sometimes it's better to make concrete offers - 'I'll drive you to hospital' - rather than saying, 'I'm here if you need me.' Housework can seem impossible if you're sore or in pain, so persuade her to let you take on some of the chores - or even club together with friends and pay for a few hours' cleaning every week.
Stay in touch
It's not always possible to visit, especially if you live a long way away. But cards, notes, texts and emails are all good ways of offering support. 'When I was really tired,' says Jane, 44, 'I didn't always want people coming round. But texts were brilliant. I knew my friends were thinking of me.' Don't let fear or embarrassment put you off. 'I once saw a friend cross the street to avoid me,' says Rose, 36, who was diagnosed with breast cancer when her daughters were three and eighteen months. 'She was probably upset. But it really hurt.'
Offer to pass on news to friends and family
It can be exhausting for her to keep everyone updated on what's going on - the results of a scan, or when the next round of chemo starts. Offer to send out news to close friends in whichever way suits her best - email, blog or text, or even a Facebook page. You can also ring mutual friends, or relatives you have in common - sometimes the people who love her most will find a phone call more comforting than something written down. Phone calls are a good way of handing out jobs, too - getting someone to mow the lawn or mend her bathroom tap.
Think up ways to make sure she relaxes
This may not be the time for strenuous sport, but gentle exercising and stretching will help to relieve tension. Find a class you can do together. Anything enjoyably girly will probably help - take round a chick flick and a bag of popcorn, or contact Look Good, Feel Better, a charity set up by the cosmetics industry to provide skincare and make-up workshops for women undergoing cancer treatment. Days out in environments as far removed as possible from hospital waiting rooms are also a good idea and can bring some normality into the situation - a bit of luxury shopping, perhaps, or just a posh lunch.
Check she's OK for money
She may not qualify for sick pay at work; if she works freelance, money may have dried up altogether. She can get advice on benefits, grants and general financial help from the charity Macmillan . Is there a friend or relative who could take on the paperwork for her.
Find out information
Details about treatments, and the decisions she'll have to make, can seem overwhelming. Offer to do some fact-finding on her behalf. The cancer charities that can provide information include Macmillan, freephone 0808 808 00 00, Breast Cancer Care, freephone 0808 800 6000, and CancerHelpUK , freephone 0808 800 4040. Remember - don't hand out advice. Just give her the facts.
Go with her to hospital appointments
She may not want to be alone if she's facing an appointment with her GP or hospital consultant - particularly if she's worried it might be bad news. Encourage her to write down questions beforehand, and offer to take notes or record the consultation in case she's too shocked or emotional to take it all in.
Talk about things other than illness
'I felt I'd become very boring,' says Louisa, 48, who was diagnosed in her early forties. 'All I could talk about cancer. I really enjoyed it when friends came round and had a rant about politics or their jobs or something they'd read in the paper. It took my mind off everything for a while.' Cancer is frightening, exhausting and overwhelming, but there's nothing to stop the two of you taking time off to have a gossip and enjoy each other's company.