in Japanese is "Tan Joh Bi Omede Toh" or "Otan Joh Bi Omede Toh". The latter is polite. But you can can get across to your friend with English word "Happy Birthday".
Just in case you want to *write* it on a card or something, the romanized Japanese writing would be: "Tanjoubi omedetou!" or "O-tanjoubi omedetou!"
Or, if you want to be very precise and use hiragana it is - おたんじょうび おめでとう。
Polite way is : Otanjoubi Omedetou gozaimasu Otanjoubi=bday Omedetou=congratulations gozaimasu=polite word
Tanjyoubi omedetou - Happy birthday to you
To write happy birthday in kan'ji: お誕生日御目出度う(御座います)--- It's usually written as お誕生日おめでとう(ございます)
how to write 'happy birthday''
Otanjoubi Omedetou Gozaimasu お誕生日御目出度う(御座います). It is very polite, but rather use the polite on instead of the casual one.