It is a difficult call. You can acknowledge the card or just ignore it depending on the situation and if you want to have contact with this person in the future.

I had a situation like yours in that it was a 15 year "friendship". The person was really a needful one and I just had enough. It is a long stroy but, the short end is I broke it off and didn't contact the person again even after serveral attempts by him to get in touch with me. I just never returned cals or cards. It was a tough break and it was very hard on me. I really needed to end it and I did and that was that.

Good luck to you. If you want to renew the friendship then whatever caused the break needs to be left alone. Don't drag it back up because it could bite you again.