Make sure you know why she hates your boyfriend, so that you know what you are dealing with. Don't ignore your best friend's comments. If she/he is your best friend then you obviously value their opinion and it is important to listen carefully to what they say. Have a good think before you do anything else. Has your boyfriend done something wrong or does your friend just not know your boyfriend well enough?;
, Ask them to talk out their differences

, Ask each of them if they would consider making up or at least not hating each other.
 For example, if your friend doesn't like him for something he did in the past, make sure they talk about it.
 If she has a valid and good reason for hating him, then there might be no way to solve this.
 You could either cope with them hating each other, or let one of them go. If this person is your best friend, it would be a good option to leave your boyfriend if you feel that your friend has a reason to hate him. Only do this if you feel that he is not the kind of person you want to be with, though.
, Just simply ask your best friend if they can try to be nice to your boyfriend. Make sure that your best friend understands that you love your boyfriend and that by being mean to him, it affects you too.
 This may be hard to lose your best friend but if they can't stop being mean if they know it hurts you, they obviously don't care enough about you.
Talk to your friend and find out what the problem is.
Introduce them to each other, and tell them that its really important to you that they both get on.
Try talking to them separately.
Tackle any specific or particular issues right away.
Consider what your friend has to say.
Consider your options if there's no other way.
If your boyfriend is being hated for no reason, don't befriend your best friend anymore, if he or she is your best friend then he or she will not put you in this situation.

,
Be nice.
If your best friend completely understands that you love your boyfriend and when they are mean to your boyfriend they hurt you too, it is time to get a new best friend.
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