^ Indifference is only one of at least two possible reasons someone might not attempt to save the person. Two others are wishing their death or putting your own welfare above theirs. The question was posed as saving your ex, not a stranger, so it is not indifference if there is hatred for the ex, but suppose it is the opposite and you don't want your ex to die, then you might still choose not to save them.
The attempt could come with harm to oneself physically or legally, and there is no way to know the attempt will be successful, nor death certain without a specific scenario. If we're allowed to introduce crystal balls into this then the ex could have just used their crystal ball to not be in the predicament where all it takes is one other person to save them.
The choice not to help could come from cowardice or from a concern for the well being of children if they end up parentless, or risking baby's death if the person doing the saving is a pregnant woman. I'm not suggesting these are good OR bad reasons, only that they are not indifference and only a single case is needed to demonstrate something other than indifference.
It would be easier to pretend that everything is black and white in life but often it is not the case. Similarly, letting someone die is not indifference equivalent to killing a person with no remorse. People are seldom indifferent to death that they could prevent, but even if they are, many will often kill without remorse if they aren't being held accountable for it, actively choose to attack which is not indifference.
On the other hand, suppose you strike someone with your car, killing them because they stepped out into traffic. There is no reason for a driver who did nothing wrong to feel remorse. Sad perhaps, but not remorse, and not indifference unless they saw the person in time to stop or veer and didn't try to.