It's perfectly normal. She'll be fine.

It happened to me coming back from Dublin last year, we were bused to the side of the terminal and entered 'land side' of the passport check and we had a Canadian family sitting in front of us but most were probably European of some sort. This was at Stansted.
When I asked the border control officer she said 'No, you didn't'. Er, yes we did, I should know as I used to work there....

I left it at that and as we walked past the exits from passport control the hall was packed, it might have been to ease congestion but we seem to be letting anyone in unchecked these days.

The UK and Eire share a common travel area CTA which pre-dates EU border controls. Citizens of either country don't need a passport to enter each others country however because of airlines id rules you do if you fly.