What you might find is that her manager but knew they didn't have sufficient proof
A manager might quickly find some other, easier to prove context to quietly get rid him for which there is proof. For example, if he's as lazy and self centred as he sounds, he's almost certainly fabricated care records which would be easy to prove: the records claim you were with so and so in such and such a place at x time. However on reviewing CCTV for that location and time you cannot be seen.
Or the boss might be looking to catch him in the act of drug taking at work to remove the question of 'when'
I doubt the boss likes it any more than your partner but ultimately she's right. There's (currently anyway) no proof of when the drugs were taken and only your partners (probably accurate) word for what happened. It's too easy for him to deny and too hard for your partner and her boss to prove.
I'd be unhappy too but I think while your partner is morally in the right her boss is being more pragmatic. Hopefully it all ends up with him gone and unable to work in the field again regardless. If not I'd be having words with unions and the care commission. Managing the situation a different way to the way your partner expected and hoped is one thing. Failing to deal with it at all is another thing altogether.
She should write out a record of what occurred and when and who was there and what was said if appropriate. As. Quickly. As. She. Can. Then date the pieces. Hopefully she can get rid of them when he leaves but they might just be useful