We have developed an iOS app that has been delivered to the customer as an IPA with an ad-hoc distribution profile that allowed a set of their employees to install it on their d
It's completely possible to take any IPA and resign it with your own details, modifying the Info.plist, bundle ID, etc. in the process. I do this all the time with IPAs that have been signed by other developers using their own provisioning profiles and signing identities.
If they aren't familiar with the codesign
command line tool and all the details of replacing embedded.mobileprovision files and entitlements, the easiest way for them to do this is for you to "Archive" the app via Xcode, and send them the generated archive file (*.xcarchive).
They can import that into Xcode so it is visible in the Organizer, and from there they can choose "Distribute" and sign it with their enterprise identity.
To import the .xcarchive file into Xcode, they just need to copy the file into the ~/Library/Developer/Xcode/Archives
directory and it should appear in the Xcode organizer. Then they click "Distribute" and follow the instructions: