When a friend is not updating, it usually does not have anything to do with your device, version, or configuration as long as you have a good connection to pull in the data.
If possible, ask your friend to verify that they have updated to the latest version of Google Maps and also see that they open up the application and select "Join Latitude". Finally, they can double-check their location reporting options in Maps for "detect location."
I believe there was a bug fix implemented with the last release of Google Maps that halted all Google Latitude updating, until a user downloaded the newest version of Maps and re-opted into Latitude. If you are still not seeing someone update, they probably have not completed this process.
If you are updating your Google locations thru a 3rd party application such as "Backitude" then you or your friends should not have seen a disruption in updates. The "check-in" you are referring to could also have been the result of a 3rd-party app, or perhaps Google's bug fix did not apply to manual check-ins. Anyways, hope this helps.
Good luck!