To say "Happy Birthday" to your friend in Telugu, you can use the following phrase:
"జన్మదిన శుభాకాంక్షలు" (Janmadina Shubhakankshalu)
Pronunciation: Jan-ma-di-na Shu-bha-kaank-sha-lu
Alternatively, you can also use the word "పుట్టినరోజు శుభాకాంక్షలు" (Puttina roju shubhakankshalu), which translates to "Birthday Greetings."
Pronunciation: Put-ti-na ro-ju Shu-bha-kaank-sha-lu
Your friend will surely appreciate the effort you put into wishing them a happy birthday in their native language.