Hi @Rawfoodworries, your 'friend' is being totally unreasonable.
Years ago, we had a beautiful girl that we were lucky enough to get at about 11 months old through a rehoming centre. She gave us many years of unconditional love and loyalty, and I hope she would agree that we gave her the same. Very sadly, when she was just about 60 in human years, she got cancer. The vet didn't know how long she would survive for, but we already had 3 one week holidays booked through the upcoming year. We didn't cancel them straightaway, because as I said we didn't know whether our sweetest girl would still be alive, and If not, we knew that we would need the holidays to be able to spend some quite time trying to recover from her loss.
As it happens with the vet's care, and steroid tablets, and other pain killers, we were lucky enough that she did survive that first year, so we just rang the holiday resorts a day or two before each holiday to say we wouldn't be coming - they were already fully paid for, so the holiday venues didn't lose out at all. Under those circumstances (and I hope your friend would do the same if she ever had that happen to her dear dog), we couldn't leave our little girl for a week while knowing full well how much she would miss us, and us knowing how much we would miss her, especially when we had such a limited time left to be with her.
I suppose the point that I am trying to make is that sometimes as parents of children, or adopters of dogs, and any other animals, we may have to change plans, no matter much they were being looked forward to. In this instance, your friend might have to cancel her plans, but that is all down to her, and you shouldn't feel any guilt over it OP. What your friend should have done was leave changing her dog's diet for another 7 weeks, when she would have returned from wherever she was going. Failing that, she should have discussed with you whether you would be ok with the new diet for her dog before she actually changed it! The fact that she didn't do either of those things, means that again, all the fault lies with her. Please don't stress about it in the meantime OP. 💐