Mail Groups on a Mac

I have a client that has a Mac and up to a few days ago he was having no problem with his email groups. A friend asked him to remove his email address from a few groups that he was in and my client just followed the usual process. Now the groups aren't editable and if the group is added to an email the group name appears but not the actual addresses of the people in the group.

If you try to edit the group the edit screen won't appear. I made a new group and it worked so the client made up new groups but they will not work.

Is there any way to fix this issue so the groups can be used?