Hi Danni, and a big hearty welcome!
There is another issue at stake here...the fact there are so many more sisters to brothers in the org. This means, if they don't marry out, many sisters are condemned to singleness
I'm afraid if your friend is determined to marry this chap she is indeed in for a rough ride for a while. Once they are married it will all settle down, BUT in the meantime she will be considered " bad association" even, ironically, by those who are also married to non-believers!!!
I find it unloving and unChristian. "By their fruits you shall know them ".