Can one determine whether somebody's work may have been plagiarized? Background

A friend of mine submitted a very original presentation proposal to the largest academic association in her field for their annual conference about 10 months ago. About 7 months ago, she heard back that her topic was rejected (according to her, it was probably because her proposal was about a project that didn't have data yet, which is nonstandard but sometimes acceptable in her field if certain other criteria are satisfied). 

She is now attending a similar conference in ~2 months. She was looking over the list of topics and found one researcher's abstract that was almost identical to the idea she had proposed. 

A little digging told us that this researcher had been one of the anonymous reviewers for the proposal-call my friend submitted to, and that the researcher has a publication-in-progress for this project. This researcher's previous work is not in line with this new project at all. 

My question

Obviously scientists independently come up with good ideas and perform similar research all the time. In many fields, research groups working independently on similar projects can even mutually support each other if they come to similar conclusions. Still, it's not always innocent. 

Are there any ways to determine whether an anonymous reviewer may have stolen ideas from one of the submitted proposals and published them -- as opposed to two researchers coming up with the same ideas independently? 

If so, how serious is this?
It depends on how identical their abstracts are. If nearly every sentence has the same concept with the same ideas, you should do further research on the details of the other person's abstract...like when it was published.