Mom of Son married to someone with FD needs help

Hello - I am new to this forum and am looking for help from those in similar situations. To summarize, something didn't seem right and I did not like the way my DIL (only a girlfriend at the time) treated my son. She treated him like a slave and is extremely controlling. He (nor I) stood up for him. He claimed he loved her - for what, is beyond me. All the time they dated she complained of one thing or another and also jumped from one condition to the next - though never had a medical diagnosis. She has gone so far now as to fake fainting spells and is trying to get herself on permanent disability, become a full time mom with major benefits from the military. We all have to walk on eggshells around her to keep the peace. My son is now completely brainwashed and is blinded to what she is doing. She has yet to have a doctor call her out on it - as she is careful to go from ER to ER and from doctor to doctor - never having them piece it all together. From what I understand - there is a lot of liability in giving a FD diagnosis and most people will vehemently deny it and refuse help. So - what is a mom to do? Lose her only son forever to a mentally ill woman? She has him in her talons and he does not think for himself or act for himself. HELP! Any advice?