The Northern Irish singer, who lost out to Andrea Begley, is helping her old mentor as the show enters the battle-round stage.
She said: "It feels so strange to be at the other side of the piano and to be like a coach adviser. This time last year I was just about to go into my battle. But it's really nice. I actually can't believe I've got this far and done my whole album now with Will.
"The battle part was the most difficult part for me because you're going up against a friend because you've got that close to them at that stage. And the thought of one of you going through and the other going home is just really, really hard to deal with."
All four coaches have brought in people to help them in the next round with Kylie Minogue getting Scissor Sisters star Jake Shears on board, Ricky Wilson enlisting Katy B to join him and Sir Tom Jones teaming up with Tinie Tempah.