I'm dating someone who has been cheated on. What should I do?

That's a really difficult question. I've experienced exactly the same situation and I have to say it's quiet hard to deal with it. It's really important to show compassion and understanding. Try to imagine you are walking in his/her shoes and how you'd try to cope with it. Think about all the emotions you have to manage and of course the pain he/she went through. Maybe this will help you getting along with it,

The best thing you can do is acknowledge it, and try your best to make them feel valued and loved so they don't feel like it will happen again.

Be ready to deal with their insecurities and jealousy, Don’t brush off their questions, Make an effort to be as open as possible. Build trust.

Reassurance. Personally, i think it means alot, remind them constantly how much you love them and really mean it.

A person who has been through a heartbreak has been heart broken and doesn't trust easily, they won't want you to take thing too fast so don't rush things. But make them feel loved and show them how loyal you are.

In my experience, it is important to maintain a healthy awareness of your partner's needs while ensuring he or she does not attempt to project a former partner's behavior on you. Communication, being open, building trust is a part of any relationship. Acknowledging the pain from a past experience is also part of a caring response. Ensuring you maintain appropriate boundaries concerning their past will allow them to appreciate a new, supportive experience. It is important to lovingly point out the differences between your experience and the negative situation when appropriate. . Otherwise, it becomes impossible to build trust.

Just be honest and truthful with them. Being cheated on can make a person really paranoid and untrusting. It's not your fault, or theirs, its just what happens when your trust is broken like that. Doing your best to be trustworthy and open helps a lot.

You nees to earn their trust. Show them that you love them dearly and make sure that they feel loved.

Be gentle with hime and honest so he can trust you. It might take him a while to trust you but he will come around.

Ease yourself into the relationship. Rushing into things with someone who has been betrayed might frighten them. And don't take this the wrong way, but ensure you have their trust at all times. They once trusted someone wholeheartedly and it backfired evidently.