Her husband knew and Doc was a cad
Upon watching the movie a second time, I saw it from another angle:
First, I see that she was targeted and preyed upon by the caddish doctor on the make. He had a "delicate" wife, which in those days means she would have her own bedroom and not have any marital relations for fear of getting pregnant again and due to lots of headaches or other illnesses women in fragile "delicate" states had in those days after having done their duty in providing an heir and a spare.
Once he knew she was married he should have stopped his pursuit. He saw she was naively ripe for his romanticizing her. Also, having a tremendous ego, he didn't want to be the only cheating one, so a married woman would be equally sharing of any guilt. It was absolutely inappropriate for him to have gone to the movies with her and beg her to see him again the following week, and then taking off from the hospital again to go Rowing- that is courting behavior, and his colleagues too must have seen him shirking work every week.
At the very beginning of the film, we see her arriving home after just parting for the last time with Doc. Her husband calls to her as soon as she comes in, he is very attentive at dinner and in the library asks her a Crossword Puzzle question that I believe he had prepared. The answer was "Romance". I believe because he was a kind and gentle man, he would never have confronted her, he was using that question as a gentle opening- for her to spill the beans!
There is no way that her husband did not notice for five weeks that is wife had changed and was going through something very very tumultuous in her life. He was quiet and reserved and boring, but nothing about her husband had him being a complete fool. When he saw her acing very strangely, he must have remembered that she told him about going with "a strange man to the movies" "a doctor".
I think her husband must have quietly found out a little bit about a romance going on, he must have known that of course it was Thursdays. The scene where she was at home after a Thursday, sitting at her dressing table, and her husband came in and asked her about her day, and then asked did she go alone to the movies?, that extra question shows he knew something was going on. She must have been acting very strangely at home before and after Thursdays, we hear her saying how odd her husband didn't notice- Of course he did! But he is too kind a person to confront her, and he loved her and didn't want to lose her by pushing her away.
I think it's clear Her husband knew the doctor (Alec) was going to end it that Thursday because he found out the doc was moving away, either from someone he asked when he made inquiries, or he might even have spoken to the doctor himself, which helped the Doc to make a quick decision to get away.
I think that last Thursday, her husband called a friend of hers to meet her at the Train Station as a favor (not elaborating to the woman exactly why, maybe just that he was worried about her health), because he knew she would be distraught and didn't want her to be alone, and he turned out to be right- she might have killed herself had that woman not been there. I don't believe that it was a coincidence that the woman showed up by accident. The fact thAT THE DOCTOR DID NOT COME BACK IN OR DELAY HIS trip and left the country SO QUICKLY shows that the husband must have known and spoken to him. Also, the Doc had seen from his friend (steven) at the apartment that his colleagues would severely frown on his behavior and his reputation would be ruined. I don't think the Caddish Doc cared about her reputation at all due to his behavior the whole time.
When she called her husband very late from a pay phone after the Apartment incident was when her husband must decided it was getting out of hand and set into motion for the end of it-He must have called her alibi 'with the sick mother" and found out it was false- he must have called Doc demanding he get out of town right away.
It shows what a cad the doctor is that he had a job offer and knew he was going to leave eventually and yet still arranged for them to sleep together at the apartment. He knew he had an escape route from the beginning but he almost RUINED her life. But her husband cleverly saved her behind the scenes.
The fact that he thanked her for coming back to him SHOWS CLEARLY that he knew.