The poor thing, I really feel for her! I mixed fed my twins for 6 months - I really think it can be done much more successfully with twins than a single baby as your body is producing so much milk.
If I hadn't introduced formula, I would have given up breast-feeding after a few weeks. I was just too tired from very little help, complications with my section, no sleep, no time to eat and drink enough,to produce the milk needed.
I had lots of people telling me to keep on with just breastfeeding, including DH who was very anti-formula. But I'm firmly convinced the girls would have had far less breastmilk if I'd tried to just breast-feed and that I was well on my way to PND from exhaustion.
I think no one can know except twin mothers how tired, how miserable you can feel and how guilty at the same time. When you've been up 6 times to feed that night and you haven't slept more than a few hours for weeks and you can't imagine life ever being any different.
I co-slept with my girls for the first few weeks but was too worried about rolling on them in an exhausted sleep. I had to sit up to feed them, to get all the pillows in place and I think that's another massively restricting factor in feeding twins, you can't just sit down anywhere/feed anywhere - you often need copious pillows and people nearby at first to take the babies on and off.
I remember so clearly feeling I needed "permission" to stop, even though so many friends with just one baby had given up much more quickly. I felt incredibly guilty the first formula feed they had, but they then slept for 3 hours, so did I and for the first time in ages I was actually full of milk by the next feed rather than crying because my breasts were pratically empty.
I also could only tandem feed, the girls always seemed to need feeding around the same time - and I found that incredibly isolating as I certainly didn't feel comfortable sitting pretty much topless in front of most people, so I could hardly get out of the house.
I think you should tell her you gave up for the same reasons - she will still ultimately make her own decision - but at least it will be an enormous comfort to her. She may well be battling along with people saying to her that you carried on and feeling that therefore so should she..she needs to know that it's okay to give up if she wants to and to just make that decision and stick with it.
It is hard that you are her only source of twin advice, but I found I felt 10x more miserable talking to friends who had just had singletons, every one seemed more rested, more together and more to be enjoying their baby than me - I loved my babies but I didn't have time to enjoy them!
She needs to rest and to start to enjoy her babies. With mixed feeding I managed to start to recover a little bit, express much more successfully, get out of the house a little and even increase my milk supply back to where two thirds of the milk supply was my own. I think mixed feeding with twins is often misunderstood and that it doesn't dry up your milk supply - there's so much bad advice around it.