That’s a brutal message with which to end a friendship, but I don’t think there’s much you can do, in practical terms, to find out if her husband is controlling her friendships.
I have a very long friendship which seemed to be fizzling out because my friend started dropping arrangements we’d made to see each other at the last minute, often on the basis of ‘So-and-so has asked me to go shopping with her, and I’d rather do that’. I thought she’d got bored with me when my health made it difficult for me to get out and about and do things with her. This upset me a lot, being dumped is never pleasant. But I then started to re-evaluate the friendship, and wonder if I had a bit too rosy a view of my friend. She was someone I thought was always ‘there’ for me, but after having arrangements we’d made changed on what seemed like the flimsiest of reasons, I started to see our friendship in more realistic terms. She is a person who likes to have a wide range of friends and the opportunity for lots of social outings, and I just wasn’t really fulfilling that role in her life.
I was never dumped by email, I’m glad to say, and as our lives moved on, the friendship revived a bit, with much lower expectations on my side!