How to improve the relationship with customer?

I have been involving in deal with big hot tommato from IQC of our important customer .

It's really a long story. Two years ago, our previous marketing manager made a good relationship with that customer as well as their IQC, unfortunately , however , anything changed, one day the guy complained corruption of boss during the bussiness as he was infuriated by our boss somehow. Thus our boss was summon to customer's headquarter to explain what happened. Our boss had no way but to request our previous quality manager to gather how many times he invited customer IQC to go out for dinner ,with who, dirty money(actually it is just some red-bag for festival),etc. and fighted back customer IQC too. So the condition is worse than worse now. Customer obviously flyed into red and complained us now and then for one cause and another .

Customer's IQC have the last word for the quality and can decide concession, rejected or Acc totally. Now they almost refuse to communicate (inlcuding the red-bag, and street-lady etc.)with us for quality issue, most of time they choose rejected to vendor, even there is only one label missing .

Any good ideas?