Ok, here’s the situation in a nut shell, your Honor. Sally inhereted $25,000. Sally met Fred. Sally and Fred opened a joint savings account. Fred transfered all the money to his own account and broke up with Sally. The former carless Fred now drives a 2002 Camaro. Does Sally have any right to get her money back? (the names have been changed to protect the innocent)
Women who fall for men who drive Camaros have no rights whatsoever.
I’ll leave the actual legal question for the lawyers, but my WAG is that by putting the money in the joint account, Sally pretty much waived her rights to have sole control over the funds. Moral of the story: if you’re not actually married, don’t give your money away.
Sally sounds like a real person with a real non-hypothetical legal issue. If so, the laws of the state where the events occured and the particulars of her situation would require that she contact a lawyer licensed in her jurisdiction and tell him the whole long story muy pronto. Many lawyers will consult with her for free.
In legal terms, wouldn’t Sally be what’s called a fucking idiot?
Yeah, seek legal advice to protect her from herself after the fact.
Sincere apologies to all for my reply, which was totally inappropriate in GQ.
I have nothing constructive to offer re: the OP.
IMHO (as a non-lawyer) she has a cause of action against ex-BF. But she will need to demonstrate that the funds were hers in the first place, that they were placed in the joint account not as a gift to him but in the expectation they be used for mutual expenses, etc. Recommendation: get a good lawyer, on a contingency-fee basis, and expect to fork over to him a fair proportion of anything recovered. Expect a long and complex fight in court.
This is advice based on things that I, a non-lawyer, heard from my mother, a non-lawyer, who spoke to another non-lawyer about a tangentially related situation in a legal jurisdiction likely to be entirely different from the one in which Sally resides so please, don’t even begin to imagine it’s actually valid legal advice. I include it here only to highlight what may possibly be some of the factors involved.
My primary savings account is actually a joint account between my mother and I. It is this way mostly because taking her name off the account, which was opened by her for me when I was five years old - would be a pain in the ass of epic proportions. She had a GIC which automatically cashed out at the end of the term and was deposited, not into her savings account, but into the joint savings account that we share. I noticed the rather excess amount of my savings (from $123 to $21,849 overnight!) and brought it to her attention. She transferred it into her own account.
We were curious and asked her banking officer what the legal situation would have been if I had removed the money from our joint account into an account that belonged to me alone and refused to give it back. There were two possibilities. If the original contract had listed another account (or no account) and the deposit into the joint account was a banking error, the bank would have simply reversed the error, leaving my savings account in an overdraft of approximately, $21.5K. If he had originally written up the GIC contract stating that it would cash into the joint account and my mother had signed the contract agreeing to that, then the money would legally and validly have been deposited into a joint savings account, and my mother would have had to rely on either maternal guilt or a lawyer to get the money back. He noted that a court might force the return of the money, but only because my mother had never actually intended to share it with me. In Sally’s case, she intended to share the money with Fred, and he just took a slightly larger share than she anticipated. The court would likely look at that quite a bit differently.
A real lawyer who actually works where Sally lives could, of course, offer real lawyerly advice, and many of them will consult with you free of charge. Go find one and ask them this question.
Sally should gather all the documentation she has of the relevant transactions and speak to an attorney licensed to practice in the state where the transactions occurred. That attorney may advise her whether or not she has a remedy against Fred.
Spoken like a true lawyer 
She damn sure ain’t getting any money back without one. 
If the legal system offers no protection I think that an outright threat to sue will often result in a settlement.