As Morrowind Mod Man said you can wait until your former companion is in a fight and join the other side, not capture her and you get 5 relation (not 4).
You can also join on her side and get relation based on the numbers of both sides (and level?).
You can also go into the same settlement she is in (hse has to be on the map) or in the tavern (she has to be in) and get a relation boost, the longer you are there the more you get (up to 60).