Taking money from mentally handicapped

In oklahoma, is there a law concerning taking money from a retarded person who seems perfectly willing to give it? the retarded person is my nephew with no legal guardian. he is very mildly retarded and had worked for the pepsi company for almost 30 years before getting fired for actions attributed to this drug taking "girlfriend". she continues to take all money he has,even his 401k money. he always gets down to nothing, not even a change of underwear, then comes back to my 75 year old sister long enough to get started again financially then the girl shows up and nephew is willing to go back with her and the process begins over again. i understan that the girl screams in his face but don't know if she threatens him physically or not; however she runs with scarey characters because i have seen some of them a couple of times. i do know "heresay from nephew" that she will find drugs and tell the seller that nephew will pay for it. i am at a loss..am concerned for safety of sister, brother-in-law And nephew.