I can relate to several elements of this unhappy situation. When our children were tots we met up with another couple with a child the same age and became "lifelong" friends with the entire family. We shared holidays, weekly quiz nights, met wider family, meals out etc etc. Suddenly after abt 30 years, the wife stopped contact with me (she only lived round the corner) and when I asked why she said "You know full well and I am not discussing it". I did not have a clue what she meant and neither did our several joint friends (nor her husband). She now has dementia and I'd love to offer support to her DH but I can't bring myself to.
I made another friend abt 20 years ago having found him researching a branch of my family tree and we were distantly related. We got on like a house on fire, emailed daily, met up with him and his wife many times though we didn't live close. Suddenly the tone of his emails changed - he started to pick me up on things I had said - one example he quoted was that I had said I was much cleverer than my work colleagues. It was totally untrue - what I had said was that I knew more about a particular ASPECT of my work than any of my colleagues due to having a personal interest in that area, but was clear to point out that other colleagues were equally expert in other areas. I got fed up of his constant criticism and ceased the contact.
In later years we went on holiday with another couple (known for decades) and whilst we had no arguments or falling out it became clear that the husband was irritated by us not wanting to be ready for the off every day at the crack of dawn. It became obvious that we weren't good holiday companions.
Then we discovered cruising and made some lovely friends, all of whom have stayed in touch and shared several more (long) cruises together, but we each do our own thing most of the time in terms of on-board activities and shore trips, even eating separately. Nevertheless, we tend to meet up for mid-morning coffee and after-dinner drinks or watching a show together. We also visit each other now and again throughout the year and phone every couple of months. We are all very different people with different life experiences (and financial situations) but we just get along well on our cruises.
Re your friends, I would ring them up - Whatsapp is great but you can't gauge a person's feelings in a message. Just say "Hi, just making sure you are both okay. Hope you eventually recovered from that awful bug. I wonder if you'd like to meet up for lunch sometime, for a catchup?" and see what the response is.