Hate is a very strong emotion and if you have to deal with a friend having this feeling towards your girlfriend or boyfriend then you have a serious issue.  You need to first of all have a assessment of why your best friend hates your mate.  You may find things that you haven't noticed about both of them.  Also you must realize that hate is a strong emotion and that there is truly a thin line between love and hate.  Did not want to get cliche but many people who hate each other are secretly hoarding the same feeling that is in direct opposite and that is love.

This is not to say that your best friend may secretly love you partner, it is just saying that a person with such a strong feeling should be considered a culprit when it comes to getting to the root of your relationship with your partner.  Your best friend is not as close a friend to you as your girlfriend or boyfriend and you should make this consideration also when making a decision about what to do about your best friend's hate for your partner.

Hate is such a negative emotion that the easiest thing to do is to dispense of such a person.  You may want to ask why is your best friend so full of hate.  A hate filled person at his best would be difficult to love because of the attention given to negative feelings instead of to the positive.  This is not to say that positivity is not the better of the two when comparing negative feelings, but most people remember the negative attributes of people unless they are saints.  That being said, you should choose your boyfriend or girlfriend because the best friend is wielding a dangerous weapon that can easily get aimed at you.

Do not try to meet when your best friend or partner is not in the same vicinity.  You will only be aiding and abating the relationship to sustain the friendship and the relationship.  You should not put yourself in that liaison sort of situation.  Because your best friend has told you that he hates your girlfriend or boyfriend. what he is really saying is that he hated your choice for a mate.  This evaluation of your choices is an analyzation of you as well because you like or love your mate.  In that scenario, your best friend is not only hating your mate but is hating the choice you made for a mate.  Some of the hate factors are attributed to you unless you choose to breakup  the relationship.

That being said, you are better off if you find a new best friend and keep the partner as your choice in which to be intimate.  You chose the girlfriend or boyfriend because you liked or even loved them as a compatible mate.  You chose your best friend because you just liked hanging out and having good times together.  Now that you have a conflict between your best friend and partner, it is up to you to make the decision of who is more important to you and to sustain your happiness on this planet.  More often than not, it will be the partner who you not only enjoy the company of but may have considered having a long term relationship.  Best friends come and best friends go, but the ultimate best friend is the mate, partner, girlfriend or boyfriend who you have the propensity to spend the rest of your life with.