Hello. I believe I can be of help to you with this issue.
It may be true that your daughter in law isn't getting help from antidepressants---only about 1/2 of people who take them do. On the other hand, it is also true that she needs therapy and would greatly benefit from it.
What you can do to further encourage her to seek therapy is to start LISTENING more closely (her husband can do this as well) to her complaints and especially, her physical symptoms. When you have a good, recent 'record' of her physical complaints, mentally tie them to her emotional distress and feelings of irritability, unhappiness. Second, locate a clinical or counseling psychologist who specializes in anxiety
disorders or better still, what is called clinical health psychology. They work with hypochondriacs all the time and typically counselors and social workers etc., don't have specialized training in this area. So make some phone calls and pre-interview a dozen or so prospective therapists on the phone and assess whether they work with patients with what are called "SOMATOFORM" disorders. Now, typical counselors won't really know what you are talking about and may make the mistake of mentioning 'psychosomatic' illness or complaints. If they use this term, you know they don't have the training or experience needed to be helpful. Or, if the can't answer the question, "What do you do exactly, to help someone who has hypochondrias is?". If it sounds like 'talking therapy' or trying to 'understand what is underlying the complaints" you again, don't have the right therapist. But if they say that they focus primarily on helping the person see the relationship between their health symptoms, the fact that their symptoms change or get worse during periods of anxiety, stress
or conflict, then you know you have someone who understands what is happening with hypochondriasis. So what you want to do is to identify1-2 specific names of psychologists you can give your son, whom you feel are pretty capable and could help.
Then, your son needs to sit down with his wife and explain that the relationship is deteriorating and that he is convinced that the health problems she is having start up and also, get much worse when she is feeling stressed, lots of anxiety, when they are having a conflict, or she is intensely unhappy. He can then explain that he knows of a couple of doctors who can help her with the stress, anxiety, depression and unhappiness she is experiencing and this in turn, is guaranteed to improve her level of physical symptoms and illness. It won't cure these symptoms necessarily, but her symptoms are having an effect on her emotional functioning and her emotional difficulties do make her symptoms much, much worse. So she can get quite a lot of help with the emotional symptoms that are tied to her health problems. it is helpful to give several examples of how her emotional symptoms are directly tied to her becoming more ill, an exacerbation of her symptoms, or are coincident with new complaints.
He needs to think about the details of this, and rehearse to himself how he will present these examples and the connection between her symptoms and her emotional distress. He can even suggest to her that he thinks her main underlying problem is a combination of anxiety, unhappiness and clinical depression---he should use these terms in describing it.
He can also offer to go into marital counseling with her; he should frame the need to go to counseling together as a 'communication' problem and his willingness to try to improve their relationship. The marital therapy is a good idea because the PURPOSE of the therapy is to fix the relationship and communication. It is more tolerable to many people because they aren't specifically the 'identified patient' or the 'problem'----the problem is a relationship problem. If she doesn't want to have any of the above offers or suggestions, he may need to then, create an emotional crisis in her life by suggesting that the current situation cannot continue and if there is no therapy, he needs to consider a period of trial separation from the relationship to figure out "what to do next". Often (but not always) this threat to the existence of the relationship will prompt a resistive spouse to seriously reconsider these very reasonable suggestions, especially the marital therapy.
I'll pause here and solicit your feedback.