In the original post, he said the right fielder became ill, which is why he wanted to put in the flex. I presumed that meant she was no longer able to play at all. If that is the case, then someone else would have to bat in that spot or they would take the out. If she could still bat then yes, you are correct she could do that.
This is correct. We subbed in our back-up first baseman for our right-fielder in the lineup. Good bat, but slower than the second-coming of Christ, so we put our DP in the field and put her on the bench.