Lost Mail

I friend has asked me to help with a MAIL app problem. They tried to open the app this morning only to find the app was asking for their email info and password in the same way as if you've never set the app up before. I guess it cannot locate the original mbox file - not sure why. Could he have inadvertently moved the file? Either way I've located the folder within his library and I can see the files exist but I just want to make sure that MAIL sees this folder rather than resubmitting the name and password info and then importing the mailbox. I this possible?

Any assistance would be hugely appreciated!

Thanks