Villain's plan (SPOILERS!)

Not exactly clear. But it seems it was, though, as she was hardly subtle about the affair, like she wanted him to find out.

Yes, except she had no intention of sticking with his best friend, he was just another tool. In fact she tells Spader ‘He’ll wish he was you once I’m finished with him’.

So it was for the money… and the sheer pleasure of destroying innocent people. She was a sadistic psychopath.