Maybe she is having depression or delusional disorder with infidelity delusions. Please consult a psychiatrist and marital counsellor

Hello there I understand your concern...For that my advice is to find out the problem they have and try to discuss with them and advice them to resolve any kind of long-standing problem. Advice them to take a vacation, and spend some time together, a new environment sometimes beneficial to couples. Advice them to do at least few of the daily activity together like daily morning jogging or brisk walking, meditation, if possible join some hobby classes to spend some quality time. If you are not satisfied kindly consult a psychiatrist for couples counseling.