As said by kts, moral and ethics vary depending on the person. What you see as 'right', others may not.
Personally, I wouldn't preform such a spell. However, we have little information on the circumstances of the break-up. If they both abide to the break up, and the relationship was going nowhere, or was abusive, etc.. then the answer is no.
I also agree with Chiron and Mercury's point. If casting this spell is to repair what another broke then it may be acceptable. Obviously, we do not know if this was the case.
I am sure if they are 'meant to be', they will end up back together in the end.