It can be a great disappointment when a new employee changes or attempts to change your relationship with what has been up to now a great customer. However you need to do (or already should have done) several things.
1. Have a reporting procedure
This can be as simple as a dedicated blog for that client, so any bugs simply get posted to that blog. In that way you can prove either how quickly you respond to bug reports or how silly some of them are, by pointing out they are based on a misunderstanding. Or you can prove no one has yet reported the 'bug' to you. Put this in place immediately for all your customers, so all bug reports are dated and visible and not reliant on email.
2. Get a thicker skin
Silly complaints, ill informed moans and blame culture focused on the outsider are all part and parcel of dealing with customers. Take it on the chin, be professional, do not reciprocate, respond in a timely and effective manner and they all pass with time. Do not take it personally because it is not a personal matter. Even if your work is below par, perhaps that is what was requested despite your protests, perhaps that was what the budget allowed, or the timescale, or the requirements at that time. Out of context anything can be complained about. My Ferrari could be a bit more powerful.
3. Never be the baddie.
Do not succumb to the level they are setting. Let them moan, let them complain, let them blame you for the ills of the world. As long as you remain professional and address each issue as it is formally raised to you, all it means is more work for you. 'It doesn't do this', 'We can do that if you want, that will cost X'. etc etc. Do not start bemoaning them or seeking rectification of wrong doing. If they want to be an arse, let them, but do not be one yourself. The chances are the moaner is moaning about everything and you are only hearing the half of it. When they get the sack and you are still there you can smile to yourself, safe in the knowledge you were the professional.
4. Never be reliant on one customer
If one customer is so big to you that if they went you would be in trouble, you are already in trouble. No single loss of a customer should throw you into jeopardy. If it does, or risks doing so, you should be spending far more time finding new customers than you are. Customers change, things change, environments change, needs change, technology changes, companies change, and your customers will change. No customer is for ever, no matter how much you like them, or get on with them. So just make sure you are not dependent on any one customer, because they will eventually leave you, and when they do it must not kill your business.
5. Love your enemy (or at least seem to)
The moaner you are complaining about, go to see him, explain the vision as it exists, take on board his comments, get his plan, buy into it, share it, tell him you are glad he is finally on board as now things can improve. Tell him how to log issues to be fixed, or ask for a new path, a new plan, a new direction, curtail to him, tell him he is right, even when he is wrong. What is more important to you, your income or your pride. Perhaps he has a point, see it from his point of view. If he truly is an arse, get him to see someone else as a target, and you as a compliant, subservient external service provider, which is after all, what you are. Bide your time. Either you will be replaced, or the other person will be sacked, or you will have to work with them. Either way, good relationships with customers are always worth fostering, even when you do not personally like them, respect their opinion or you think they are wrong. As long as they pay your bills, quite honestly, what do you care?
Yes it is a shame when a great customer relationship changes, but they always do in the end. Milk it, raise above it, and if needed, move on. What more can you possibly do?
