iOS Release IPA using someone else's .p12 & .m

I'm trying to build & release an IPA and sign it with someone else's (The customer) .p12 file and a .mobileprovision.

What I do not have is their apple developer account user and password combination (I only have my own user and password which are not related).

How can I build and release it using their certificate which I mentioned before (enterprise distribution)?

I'm using Xcode Version 5.1.1 (5B1008)

I have imported the p12 file, and it appears to be fine and visible in keychain. In Xcode, I have set the correct bundle id, and under "Code Signing"->"Release" I have chosen their imported certificate with came with the p12 file ("iPhone Distribution: xxxx"). But when I click Product>Archive, build fails with message '.... no provisioning profile matching the identity "iPhone Distribution: xxxx" was found.' (I put the xxxx there for anonymity).

Note- Before I'm flamed with "Possible duplicate of..." comments, please know that I have looked into other questions and could not find a working solution.

Thanks.

You have to import the .p12 and the related provisioning profile properly.

Delete your existing certificate from Keychain. Open Keychain Access Manager. Navigate to File > Import Items.
Browse to the .p12 or .pfx file that you want to import and open it.
Select System in the Keychain drop-down and click Add.
Enter the admin password (if Asked) to authorize the changes.
Enter the password that created by customer when he generated the .p12 file and click Modify Keychain.
Next, Remove your old provisioning profile and get & add the .p12 related provisioning profile.
In Xcode, Under Target-->Build settings-->provisioning profiles, select the correct provisioning profile from the pull-down menu. Make sure that you are not doing it under the project section instead of Target.

Hope this will work fine.