There's a word for someone that betrays, eg. the traitor and I wondered, is there a specific, single english word that means, "someone that was a victim of a betrayal"? So something that isn't broad and can be used for a variety of scenarios such as victim.
Someone who has been betrayed can simply be referred to as "the betrayed".
This is similar to how you would say "the deceased", as in -
- Half the services I've been to, the presiding reverend never met the deceased and still managed to say something nice.
Therefore, "the betrayed" works just fine. Taking David M's example from the comments -
"The betrayed often find it difficult to trust again."