Starting at 7th level, the paladin, as well any fiends and undead within 10 feet of the paladin, gains a bonus to melee weapon damage rolls equal to the paladin's Charisma modifier (minimum of +1). A creature can benefit from this feature from only one paladin at a time. At 18th level, the range of this aura increases to 30 feet.
There's nothing in here about only affecting friendly targets. So an Oathbreaker fighting some undead or fiends with the rest of his party will actually make them more powerful from his/her presence?
This is assuming the player has been granted permission to play as an Oathbreaker by his/her DM.