There are always two sides to every story. The wife may believe she has never said anything wrong to her mil but maybe her body language and general attitude says different…
When you make the choice to marry someone of a different faith and/or
sect, you have to be strong enough to deal with the consequences, and those consequences may very well be never having a cordial relationship with inlaws. If they didn’t approve of you, and still don’t, then the best you can do is remain respectful on your part and go about your business. In one ear and out the other.
In “general” terms, if you’re adult enough to have a love marriage with someone of different faith, shouldn’t you be adult enough to find a way to coexist peacefully with your inlaws?