what's the best thing to do in this situation?

a bit unclear there in your wording, but speak to her calmly, and don't let her interrupt until you say what you want to, to clear yourself from this. Then tell her what you know without hurting her feelings, afterall she needs to know what you know. She will believe you and trust you then.

Evidently you've not done something right for the other person to think you've done somthing wrong. Attention and having fun and conversation is important to some people. Make this person feel and show them that they are the one that should have your time not your so called friends or co-workers. Take that person out and have fun more often, Take no phone calls while you are alone. Better turn the phones off. Take them to meet your friends, be apart of your friends circle. Involve this person into your life.

Personally I would confront the individual that accused me and ask her whom said it and then the two of you confront her or them and see how this started A lot of times people only hear what they want to hear and If I said my friend went out the guys and had a wonderful time with the guys by the time it was repeated by 10 people the story changed and he is unhappy and went out to find someone else because he doesn't Love her, Believe none of what you Hear and Half of what you see, and I think your friend will understand this.

You have to defend yourself. That means defending the current situation. Don't bring up other things especially from the past to defend yourself. That's the biggest problem that people do when defending themselves. Just stick to the current situation and be firm. You can't just do nothing but you also don't want to escalate the situation by bringing up other things.

you should talk but not in an offensive or mean way just make sure that person is happy and so are you just make the best of it and dont lie no matter the situation

Not enough info.

IF YOU DIDNT DO ANYTHING WRONG YOU DONT NEED TO DEFEND YOURSELF.

whatever you wanna do