I'm another one disbelieving of the 3 hour turnaround for a deliberate overdose, I'm afraid, @midlifeangst. My daughter has diagnosed MH issues and is prone to claiming to have overdosed on her medication before now - 3 times, in fact, unfortunately - and the minimum time spent being observed/waiting for the psych team to assess her was 14 hours. And this was pre-Covid, when A&E departments weren't slammed with 111 callers advised to go straight there for minor things.
Unfortunately, it does sound as though your DIL is being emotionally abusive/manipulative of your son - by claiming to have taken an overdose and attended A&E alone as a result, she's sent your son into an abyss of panic... and he's gone rushing back to take care of her/be with her. She has him exactly where she wants him.
Only the first of my daughter's overdoses was genuine, and taken because the young man she thought she was going to be with forever and ever, had dumped her for being irrational/possessive of him. It was a desperate attempt on her part to get him to "un dump" her... and it worked (sadly). The second time was a pure manipulation, again to convince this young man to take her back after she'd assaulted him (thankfully, he refused point blank), and the third time was to try to manipulate the police into not arresting her after she'd assaulted me. Both of the latter times, her blood tests showed no traces of an overdose in her system, and the time was spent waiting for the psych team to assess her. She's now stopped this behaviour, but only because it was spelt out to her that she was very close to being locked up for the abusive manipulation of it all.
Your son needs support, if only to help him to see/understand what your DIL is doing. Maybe he'll choose to walk away from her - but you have to realise that chances are, he'll stay with her. And you'll have a choice to make, I'm afraid OP - support him, or walk away from this situation yourself. It's not an easy choice to make, either way. I chose to support my daughter, despite what she's done to me over the years, because she is ill and I'm her mother. If she were my spouse, however, I know I would walk away (I did with her father). I've lost family members and friends, however, because of the choice I made - and I suspect your son will, too, if he stays with your DIL. It's not easy, there are a lot of ultimatums in place (ie, medication is not to be abused in any shape or form again if she wants me to stick around), but she claims to be deeply ashamed of her irresponsible/immature choice to claim to have overdosed in an effort to control someone else/manipulate a situation to avoid getting into serious trouble. But this comprehension came about after a lot of therapy/counselling.
Could you suggest to your son that he seeks counselling for himself? I know a lot of therapists have helped "victims" of emotional/physical abuse to realise that they can walk away and not be responsible for the choices their abuser makes, because my daughter's ex was one such individual. If he stays, expect the manipulative behaviour to ramp up and your DIL dig her control over him ever deeper. It will get worse, I suspect, before your son sees the proverbial light. And as his parent, you'll feel like your heart is being ripped out every time your DIL successfully controls/manipulates him into allowing her to get/have her own way. Can you support him through abuse ramping up? Into admitting that his wife's behaviour is abnormal/abusive? Into seeking therapy? Into leaving his marriage? Because it's not an easy path to tread, OP; one not for the faint-hearted.
DM me if you like. 