Every successful relationship all started with some kind of flaws and imperfections because as humans we can never be perfect. Everyone has his or her weakness(es) which sometimes reflect in their way of life.
I believe marriage is a bond that joins two people who are in love with each other together. This implies that when you truly love someone, you will be ready to make sacrifice for them and you will always find a means of not loosing such person because of the love you have for that person.
From your question, it shows that the lady (your gf) truly loves her husband and she doesn't want to loose her husband out of the way her husband talk rudely or impolitely toward her. I will advise that she tell her husband with respect and a caring attitude when her husband is in a good much such that she won't make her husband angry while correcting the flaws.
A good husband must always respect and care for his wife. He should also be able to make amendment t on his mistakes whenever he is corrected. This also applies for women. If she notices her husband is getting angry while correcting him, it is preferable for her to just keep mute to avoid increasing the anger of her husband.
Also, it depends on the mood of the husband due to the kind of bad experiences or stress he might had undergone which the wife might not know about. The husband then tends to vent out his anger towards her or make his wife frustrated as a means of relieving himself from blame. It is advisable to always check on your husband. Motivate him and increase his hope in life and make him relax anytime he had undergone stress or bad experience as this will make him feel more comfortable and relaxed.
In conclusion, it is just a matter of time. I believe with time they will both begin to understand each other. But it is very important she tells him about it in such a calm and respectful manner since that is the only thing that she doesn't like and I am sure her husband will surely work on it by eradicating such behavior because "he won't want to loose what he loves (his wife)."
Thanks for reading and I hope this helps.