Reminders no longer separates items by line
I have tested a list just like you specified on iOS 9.0.1 (13A404) and used the notes app to make a note with the three items, one return, no special characters, spaces or line breaks added between the three items.

I then long pressed and selected all the text in the notes app, copied it, went to reminders and pasted it. Use care to paste into the list itself and not to paste into an empty reminder.

The behavior for me isn't a change as I'm getting multiple list items and not a list of three items.

If the above doesn't help your results, you might need to look in to the specific app that your wife is doing the copy operation to see how she is getting the text and double check exactly where it gets pasted. If it's a long enough list, you might try pasting it into notes.app to see if that helps or use notes.app to make the list in the first place.