One of my friends says it was in 3.5 but i can't find them in any books.

IIRC, it was a template in 3.5, but I don't remember exactly which book it was from.  In 5e we have Aasimar as the closest version.

If its 3.5 then its no longer official, its obsolete.

Half-Celestial was template in the 3.5 Monster manual and Lunali, the Aasimar has been around since 2nd edition. 

Half-Celestial, Half-Fiend, and Half-Dragon were all templates in 3.5 that could be applied, with DM permission to playable characters. 

Other favorites include Half-Golem, Half-Giant, and other fun templates like Tauric Creature.

Monster Manual I, page 144.

A half Celestial is considered an Aasimar in 5e, an half Fiend is a Tiefling. Half Dragon is a template in the MM, but WotC has replaced it with the Dragonborn as a PC option for 5e. We also no longer have the half Ogre (Ogrillon) as a PC race, in my campaigns I reckon Goliaths for their stats.

Quote from Chaosrex >>

If its 3.5 then its no longer official, its obsolete.

 Not if you still play 3.5

Then you won't be using dndbeyond :P