For internal testing we want to install an old version on a device of someone who's UDID is not in the list. Via TestFlight I can add the email of this person in order for him to download the newest version (via internal testing), but I was wondering if it would be possible to enable him to download and install a specific old version?
I've seen this page: how to install previous version on TestFlight but it is rather old and the suggestions are incorrect.
I found a workaround to test older builds with TestFlight. You can just use the link provided in the email you received when you uploaded the older build for testing. You will see something like this.
To test this app, open TestFlight on your iOS device using iOS 8.0 or later and install the update.
Tap on "TestFlight" in the above line. This will open TestFlight with a link to the older build. And you can install it if it hasn't yet expired.
Hope this helps!
Update
As of April 11, 2017, TestFlight now supports multiple builds. https://developer.apple.com/testflight/release-notes/
You can navigate to the Previous Builds section of the app in TestFlight and get access to builds which are up to 90 days old.