Hi - I am fully bf-ing my twin girls, nearly 13 weeks, growing nicely. I'm no expert but maybe my experience so far will help in some way?
Recently I have also started to think about introducing formula as I am really tired and I have lost over a kilo a week since they were born (after the initial 7kg drop at birth) - yes 22kg so far! I tried expressing and found it relatively easy but it doesn't reduce the physical tiredness.
I am finding it really hard to think straight about it as in my head I was hoping to bf exclusively for 6 months, and I love it, and my girls are sooo easy - quick feeders, sleep well - but still I am sucked dry physically. It is hard emotionally too because I think bf is making me feel like I have permanent PMT. BF has got easier in the last few weeks as I have cut down the number of feeds gradually to 5/day. But still, I feel drained.
My mum came to visit this week and she suggested I consider mixed feeding so I can have some decent breaks for sleeping and/or going out and having some time to myself. She told me she bf-ed me for one month then felt too tired and switched to formula and I am fine, really strong immune system. Just talking about it with her really helped. Having someone say 'you don't have to carry on if it's not for you, your babies are unlikely to suffer, you're not a bad mother if you formula feed' was great.
I also realised that one reason I haven't used formula yet is that I am exhausted by even the thought of having to find out about bottles and sterilising and formula, let alone having to do it! I am thinking about 1 or 2 formula feeds a day, but also expressing to keep up my milk supply so I can change my mind if need be. It will be more work initially so I will probably rope in DH to work out the formula side and do the bottle feeds initially.
OP you sound like a great friend and I hope you and your friend will be able to talk through all the options and maybe how the transition could be done too, if she decides to mixed-feed or to stop bf entirely. If she wants to keep bf-ing, it WILL get easier.
I really like the 'it's not poison, it's formula' post!