I decided to reach out here rather than fretting by myself.
I'm glad! It's a lot to try and sort through these baffling and harmful behaviors without support.
So far his partner has lied to him about interactions each one of us have had with her and in every case the lies cast us in a bad light - such that my son felt the need to speak to each of us about our perceived negative behaviors. It is not easy to relay what is going on because she is very clever - in all instances the situations actually took place but the contents of the conversations were twisted or did not happen at all.
Your son probably knows this or is catching on. After 16 months dating her he's likely been on the receiving end of the same thing

More likely he's now in a loyalty bind and trying to run interference to appease her. Every time she says HardyVine said x, and he chooses her, he's passing a test.
One time I let SD26 know I left cash on the kitchen counter so she could grab dinner or order to-go. I was going to be on campus working late and wouldn't be home to cook. She texted H that I didn't want her in the house.
Another time, coming into the kitchen when she was there with her BF (two socially awkward people), I asked if they had plans for new year's (that night). Later this became "LnL was asking prying questions about our sex life and it made me uncomfortable."
To a certain extant I realize that I can't really say anything to him about her that he would be willing to hear and that I need to resign myself to the fact that I can't protect him from the suffering that I anticipate he will endure in this relationship.
That's both wise and heart-breaking to hear.
How does the rest of the family interact with him? Do they experience something similar?
I have not said anything to him directly regarding her deceptions because I feel this would drive a wedge between us.
How do you engage with her when he asks you to address her (deceptive) comments?
I am also fairly certain that she has relayed things to me that he has said that are not true.
The details of those things could provide some insight into what's going on for her and how she's perceiving reality (about him). There's a lot of projection likely going on.