Sister-in-laws Ex-bestfriend Is Threatening Lawyers/court If She Doesn't Give Her $5000

Location: New Jersey, USA

I'm not really sure if this is the right sub to post under, but we are all kind of at a loss in this situation.

So my (22f) Sister-in-law (21f) is dealing with harassment from her ex-best friend regarding financial payments.

The story goes like this: they were best friends for about 5 years. During this time, they took a few trips nationally and internationally. The deal was that her friend (let's call her D from here on out) would book them without my sister-in-laws (B) knowledge and tell her she had to go with her. So B was only working part-time at the time and could not pay her back for most of the flights/airbnbs they stayed in. However, she paid for most of their activities and food wherever they went. B agreed to pay D back when she had a better job and could afford it.

Around this time last year, D decided she didn't want to be friends with B anymore (this is a long story that has nothing to do with the legal advice I need). D then asked for all of the money back for the trips. Which is fair.

So B started making payments to her. They didn't originally agree on an amount, but B had all of the paperwork and was able to determine that the trips they took were equivalent to about 8000-10000 for BOTH of them. Since she was only expected to pay for half of the trip prices, she determined that she would not have to give her more than 4000-5000. However, her and D never had that conversation.

As of today, B has given D over 4000 total over the past 10-12 months. Recently, D reached out and said she still owed her and would "crunch the numbers" and get back to her with an amount. B agreed and just said let me know with what you come up with. 2 months later, D comes back with "you owe me well over 5000 still, but since I'm nice, let's just say you owe me 5000. You can pay me whenever you want, but if you don't respond, I will be talking to you at work and then I will take you to court."

Again, B came up with about 9000 total for both girls for the two big trips they took. And yet D is asking for 9000 total just from B.

I guess my question is, legally, what steps should we take her? Can D actually sue her for that money? How do we make this a less hostile situation? I'm just so lost and worried. It's causing B an unnecessary amount of stress and we don't know what to do. Any advice? Any additional information that is necessary, just ask me for it and I'll try to answer to the best of my abilities. And if there's another sub I can post this to, please let me know as well. Thanks!