Gradually.
That was the secret, wasn't it? Build up their trust or indifference, either one would do, like in that fable about the mouse and the lion. First it was hello over the mail as they each made their way back to their separate apartments, next? Why, after months of chit chat, it was coffee shared in the buildings laundrymat. More chance meetings, more talk about incidentals, info on her fake bio. There was no need for him to learn of her unpleasant past. He would only get the wrong idea.
It wasn't really lying, not when she was honest about the things that really mattered...
She was careful too, set up blogs and filled out online questionnaires so when he inevitably googled her name, she'd be believable. Trustworthy.
These things took time.
As she slid the key he'd entrusted to her in the lock, she knew she could give him an excuse that would be believable. Dropping off the mail? That would do.
She made her way into his apartment the way as she had made her way into his life. Gradually, under false pretenses, but with the best of intentions. When his wife got home, she'd be waiting for her.
Gradually.