First: Talk to your daughter. Tell her that the aunt has no right to harrass her, but unfortunately nobody can stop her from phoning. Tell her that there is nothing the aunt can do to hurt her. Tell her that there is absolutely no reason to worry about the money that the aunt wants, because there is no way that the aunt will get a penny. Not from you, not from your wife, not from your daughter.
Second, in case nobody thought of it, check whether your daughter's phone can block numbers. It's quite likely that she can block her aunt's number. The aunt might try to use another phone, that can be blocked as well. If she can't block numbers, get her a better phone that can. Which will cheer her up as well, and I guess she needs that.
Third, you need to find out what is exactly going on in these phone calls. So when you're at home, she can unblock the aunt's number, and if she gets a call, come straight to you before answering, then take the call with you present, and put the phone on speakerphone. Prepare some question your daughter needs to ask: Why does the aunt want $8,000? Why does she think the daughter should give her $8,000? Why does she think the daughter has $8,000 to give her?
With that information, it will be easier to handle this. Maybe there is a rational or irrational reason why the aunt thinks she is owed money. Maybe someone told her your daughter won the lottery, and she figured out she wants a share. Maybe she needs money and your daughter seemed a good victim. Maybe she just wants to upset your daughter. Anyway, once you figure that out, you can go from there.
If you can't find out anything, then you need to assume that the aunt has gone mad. Your wife might make enquiries first in the family whether the aunt is alright. It might be the beginning of Alzheimer's and someone, years ago, owed her $8,000 and she thinks it is your daughter. Thinking of madness is extreme, but so is the whole situation.
Good luck. Once more, tell your daughter not to worry and explain to her why she doesn't have to.