Sorry, I disagree with Ella. You have a right to go on Mat Leave and return on any conditions you and your boss agree to. But you have no right to control who does your job whilst you're away or the conditions they work under. If your boss agrees to her coming back part-time, you can't change that, and it's actually none of your business.

Also, I don't see that she has been underhanded or snaky. She's entitled to enquire about job opportunities available and apply. It's actually none of your business - and HR will likely tell you that. If HR and/or your boss know how to do their jobs, they won't even discuss her application/conditions or work with you. And to be honest, if I was in her position I wouldn't want it discussed with anyone either.

Sorry to be blunt, but I don't see that you have anything to worry about. It's your job, she can't take it from you. That's all you need to know.