I am sorry for what happened to your friend that is terrible, however things only happen for a reason there maybe a testimony for her to give in the future. You have to understand that blaming God for things happening that are bad to someone you love or care about isnt going to change the fact that it did happen. Blaming even if not aloud the siste in law for what has happened also does not change the fact that it happened and really it wasnt her that did it.
Personally, many people have said bad things to me like " you will never have kids", " you will never have anyone love", " you are going to die before you are 18". Guess what i have three girls, a husband, and im 23, one reason i believe none of those bad things happened to me is because i gave it no energy, no time, nothing. I put i in my head that it was just words nonsense and nothing more.
I believe you give somethig even the smallest amount of energy or some kind of belief in it, it can begin to manifest itself. But on the other hand it may not be the case as i said before things happen for a reason and things always get worse before they get better, and who knows God may have a huge big purpose, giving her the testomony to be heard by all, of how she went through a rough time but God brought her out and now she has so much to share of how she made it through.
You could tell your friend to have faith this obstacle will pass, it is just another hurtle to jump over and sh can do it. She will get through it, it just takes time. Maybe even suggest that she try to mend things with her sister in law it may help.
There is this song i like and the words that stuck with me the most is, "we need to start looking a the hands of the time we've been given this is all we got, we gotta start thinking that evey second counts on the clock thats ticking gotta live like were dying, we only 86400 seconds in a day to turn it all around or we thow it all away we gotts tell them that we love them while we got the chanc to say gotta live like were dying". Please have faith and tell your friend the same.
May God bless you all!