What is the reason for the recurring nightmares that afflict the wife, and what is the solution to this problem, knowing that she did not suffer from them before marriage and is religiously committed?

We have no knowledge of the cause of disturbing dreams, and there is no cure for them except for remembrances (dhikr) and lawful incantations (ruqya), along with realizing reliance upon Allah and sincerity in seeking refuge in Him. A trusted psychiatrist can also be consulted.