If there are only men or a mixture of men and women then it's "Eles estão felizes". If there are only women then it's "Elas estão felizes."
The correct way to say Happy Birthday to someone who speaks European Portuguese is as follows. Say "Feliz aniversario." That means Happy Birthday in Portuguese.
You say "feliz".
"Feliz dia da mãe."
"Happy Anniversary" in Portuguese is "Feliz Aniversario", the same as in Spanish.
Feliz.
Feliz Aniversário!
Felizes Férias.
Felizes Férias.
Feliz Aniversário
"feliz aniversário"
Feliz Sabado
Feliz aposentadoria.