Can't get insurance co. to consider claim (Medicare related)

Hello, just like everyone else I have been looking to the internet to try to find a solution to an insurance problem. I've done a search here but can not find a similar case so perhaps the best thing to do is lay it all out. Thanks in advance for any tips or experiences you may have. My fiance's mother spent a long time in a nursing home before she died in March 2006. Medicare sent the patient a summary indicating benefits would expire (I think it has a 100 day limit) in a certain number of days from the date of the summary. Accordingly, the nursing home did not bill Medicare after the benefits expired and the patient paid the bill (a little more than $7000). United Healthcare is the secondary insurer but refuses to consider the $7000 claim because, they contend, they are responsible only for charges not paid by Medicare and there is nothing in file to show Medicare denied the charges. We told them the nursing home did not bill Medicare after they notified them benefits would expire after a certain date, therefore, there is nothing from Medicare on the specific dates of the claim. UHC insists that it must have a rejection from Medicare. We contend a rejection can not be obtained because the nursing home is not going to bill Medicare for charges that do not exist (they were satisfied by the patient). We are thinking of going to Small Claims but wonder if we have a chance. Does anyone have any advice?