I actually tried couples therapy with my husband, a covert narcissist. It did no good for our marriage, but it did validate for me, even more so, what was going on. He threw me under the bus to the therapist, saying things I had never heard before, taking credit for things I had done, and causing me to look pathetic if I objected. Everything was my fault and my responsibility to fix. This became very clear. It was so eye-opening and validating to me that I was in fact dealing with covert narcissistic abuse. I could no longer deny it at all. However, it did nothing to help our marriage.
Covert narcissists use therapy as a way to justify their behaviors and to turn the tables against you. They manipulate anything you say as well as anything the therapist says. I have seen this myself and heard it from many victims too.
If the therapist is not educated about covert narcissism, marriage counseling can certainly work against and actually feed the narcissism. They often charm the therapist and convince them that they are doing all they can to help this marriage. This often causes the victim to get quite frustrated and reactive, giving the therapist even more reasons to believe the narcissist. The therapist then tries to work with you on how you can communicate better, handle your reactions better, and so on.
When you have been pouring years of intense effort into this marriage and then get told, in front of the covert narcissist, by a therapist that you need to do more, this is maddening. It can often cause the victim to further doubt themselves and pour even more of their quickly dwindling energy into the relationship. This just fuels the covert narcissist even more, as they will continuously throw the therapist’s words in your face to remind YOU of your responsibility.
The only way for couples therapy to be effective is for both people to be willing to carry some of the blame, reflect honestly on their own behaviors, take ownership of their own faults, and be genuinely sorry for ways they have hurt the other person. This can happen in non-narcissistic relationships but is not possible in narcissistic relationships.