Can a PI take someone's idea
As far as I can see "member" has developed everything from idea to finished product. There is very little sign of anyone else being involved other than possibly as a discussion partner at the lab. To allow someone to use the idea still does not take away the intellectual property held by the member unless there has been important feedback from someone. So I would not hesitate to try to move forward towards publication with the idea. I simply cannot see any ethical issues based on the details you have given.

Having given the green light, there will always be the issue of personalities. You allude to some level of conflict in terms of the move by the member. It is not inconceivable that the PI may have a different view and this view may not even be anchored in reality. So even if you do everything by the book and have all rights, that does not preclude the PI from doing all the things you express, it really depends on the PI's personality.

In the end member should think through if anyone has contributed to the work to the degree that merits co-authorship. Member can, for example, use the the post What are the minimum contributions required for co-authorship to set the authorship in perspective.