Having a conversation with your customer would most likely be your first move.
Sometimes nuance and certain hints in conversations can be missed. If the person has someone else to answer to, and he or she is on the spot, the person might go into "flight mode" rather than fight for you.
Here's what you need to think about:
Is this relationship a major account that I need to save?
Is this person that I thought I had a great relationship with worth working with? If so, what do I need to do to find out how to help them repair their relationship with his or her superiors?
If I let this go, will it wreck any other relationships that I have in the marketplace? Can I repair them if they do get compromised?
How much revenue do I have to generate if I do let this go?
Businesses are built on relationships. If this relationship was really that strong, would this person have done this? Does he or she have any intention of communicating with you to get things fixed so you can get that business back in line?
Once you realize the value is in the relationship rather than the account revenue, you'll be able to figure out whether or not the account is truly saving. Then you can plan out your actions from there.
Find the value in the relationship and then find the value in the business.