Freemailers allow emails with unusual parts before the at sign. Like ...email@example.com which looks cool with certain short names, but which is also rejected by many mailing programs. Another tricky one is an abbreviation like a.b.c.@gmx.net which is also valid there, rejected by some strict checkers, and most horribly often mistyped because people also usually think there should be no trailing dot.
Lessons learned? Not to check too strictly (like said above) if at all, and secondly both friends got alternate addresses without any dots before the at, just to be sure everyone could message them.