Is it okay that my boyfriend has his ex's phone number?

One time I had to use my boyfriends phone to call someone so I asked him if I can use it and he gave it to me and I was looking for the contact in his phone (side note: he puts hearts next to some of the names like mine and a couple of other friends which I'm okay with because they are also my friends and I love them) but I saw he still had his ex girlfriend's contact with a broken heart emoji and I heard him mention to his brother before that she had broke his heart and before we started dating he was scared because of how his previous relationship ended. But my question is if she hurt him like he says she did why does he still have her number and they are also Facebook friends. So should I be worried about this?
Is it okay that my boyfriend has his ex's phone number?
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