Husband and his long distant female friend

Been married for years and my husband has been friends with a women for 15 years but they only knew each other for two weeks when they first meet. I have always known of her and we have posted Christmas cards to her and I think once or twice we bought her small gifts in the early stages of our relationship. Over the years I had thought they talked once or twice a year on a "how are you and what are you up to" basis. I seen pictures she has sent of her and past boyfriends and never really thought much of it. The last few my husband grew angry and a bit distant. He said he was unhappy at work and said he only had fleeting moments of happiness in our marriage. I was worried and thought he was depressed. I was always working but never earring as much as him. His wage is 5 times more than mine. I always felt bad about this as I struggled to earn more for us as a couple. It was like we were fighting over nothing for a year or two and he said we have not been happy for a long time and that we fight all the time and that I am not being honest. Then my mom got ill. I thought she was dyeing but she has been ill for years we have always thought this might be her last year. So I went to the country I am form and visited her. Shortly after returning back my husband had contacted this women friend and she announced she was coming down to see her brother and to visit and could she stay with us. As I was a mess with the stress of my mom and our relationship I asked for her to stay at her brothers and for us to meet up. My husband thought I was being selfish and told me so. We argued for months over her staying. I still didn't know if my mom was going to make it. And as it turned out she went into hospital days before his friends arrival. I was also changing my medication as I have tumours and so I was having side effects. The visit went badly I was so confused and was not very friendly but I did ask several times if we could just meet up instead of her staying but he was insist ant. After she left he didn't talk to her for months and we went to counselling and days before our anniversary he says he wants to contact her. So at first I am not happy about it and then I say okay. But when he contacts her she said as I don't like her and I acted odd and had trust issues with their friendship she had given up on their friendship but if he wanted to send cards and emails that was okay.
He was hurt she gave up on their friendship. And we argued about it. I said I would sorry and explain my behaviour (husband says I don't have to call her and explain)but I am afraid I am asking for trouble by opening the door. She knew my mom was unwell at the time and she didn't even ask if she was alive.