I'm a recovering master rugsweeper/gaslighter.
I could believe it if he had PM'd other people but a woman, who looks like OW, knew he sent it, knew it when you confronted him but didn't come to you beforehand???
Even I'd have trouble rugsweeping/gaslighting myself into believing that it was the ambien.
I don't buy it. Not for a second. It wasn't an accident, it is a massive, flaming, flashing red flag.
I'm so sorry honey.