I am finding it difficult to advise you on this question. If Adult Protective Services conducted its investigation and found it could do nothing, then it seems likely there is nothing you can do, either. Your only remedy would be to establish a physical conservatorship over the senior citizen, and have him moved out of the home. If the senior citizen is capable of taking care of himself, however, the court likely will deny the conservatorship.
A similar analysis applies to the financial claims. If the senior citizen chooses not to pursue the claim and has the mental faculties to make that decision , there is little that you can do. The only thing an outsider can do is to attempt to obtain a conservatorship over the estate of this senior citizen -- and that attempt will fail if the senior citizen is indeed capable of managing his affairs and simply chooses to be sloppy about them. In addition, a civil action against the friend sounds like it will ultimately be pointless , if the friend does not have the money to pay a judgment.
If you are writing on behalf of the senior who is seeking help to get this "friend" out of his house and recover his money, then you need to meet with a lawyer in your area, meet with someone from a senior advocacy agency in your area, and otherwise tap into local resources to start appropriate ways to evict this "friend" and protect the senior citizen.
You can make a new complaint to APS and don't leave out the important details this time like the missing $600,000. Failing that perhaps you can find a relative who is interested ijn intervening. You can also speak to laew enforcement and see if they have any ideas especially if the perpetrator is known to them.
This is not legal advice but a general comment on society. International Law 24/7 hotline +1-202-318-2406 - Dr. Jonathan Levy, PhD calls or emails usually returned within 24 hours.
Your senior friend needs to speak to an attorney who specializes in financial elder abuse. APS and the proper authorities are always the right way to go. However, if you believe that the defrauding individual still has your friend's money, then civil action should be taken.
My firm has handled cases such as this, if you allege that the fraudster was a corporation, there may be alternative avenues in recovering the lost funds. For example, the Victims of Corporate Fraud Compensation Fund may be an avenue for your friend to recover at least a portion of his money.
I urge you and your friend to consult an attorney to determine what options he has.
This is not legal advice and does not intend to create an attorney-client relationship. You should consult an attorney if you have any questions regarding this matter.