That's a bit different than claiming time theft, isn't it? That's more of a suspicion that someone couldn't possibly be doing a good job if their attention is split.
The two full-time companies objected strenuously to her leaving and made various offers to keep her around. Would they have suddenly decided that they were deluded and that she was actually a horrible employee if they knew the truth? Possibly, but that doesn't mean they're in the right.
The real problem here is that a large portion of managers actually have no idea what constitutes a good employee or good job done. So they try to figure it out through second order observations like how many hours are worked, how convincingly the employee acts like they're good at doing their job, or how they report on their progress in meetings; and they get upset when they find out information that makes it seem like the employee should be doing a worse job.
I'm being a bit harsh, since I'm not immune to the same thoughts and uncertainties; I have an employee of my own who has two other part-time jobs and at times I've questioned whether that's OK. When I really examine the work he's doing, though, the answer is always the same: he's doing a good job, and the rest of his time is none of my damn business.