This was a relationship that evolved. I think at first the relationship between the two was not always smooth and amicable. Queen Marie was (IMHO) a character and one of those larger than life people who demands center stage and attention. Queen Mother Helen was a more sedate individual. It was probably like oil and water at first.
On top of that, Helen was pregnant early in her marriage and not happy and supported by her husband. Surely Marie realized how awful a partner her son was.
Certainly after a few years, Marie came to be appalled by how Carol treated his wife. I believe for the rest of their lives, there was sympathy and respect between the two ladies. It was not likely a friendship as there would be from two people who enjoy the same interests or who are alike in character. But, certainly there was respect, and probably love, too.