My friend Henry didn't go to class yesterday, and neither did I.
Literal
Friend [appositive-の] Henry [topic-は] yesterday class [to-に] did-not-attend [but-が], I [also-も] so was.
私もそうでした ('I was also like that') is the standard 'me too' / 'neither did I' construction — そう refers anaphorically to the previous clause. The appositive の in 友人のヘンリー identifies Henry as the friend.