Is she expressing any confusion or apprehension about wanting to go to mass? Or has she just said that she wants to go? My opinion would be to let her make up her own mind on this. Maybe she's enjoying the companionship she's finding with your group and that's why she wants to go on outings with you. Or, maybe she's dissatisfied with her current faith and is interested in experiencing your faith. If I were you, I would leave it up to her. You're not putting any pressure on her to go to mass with you. She's the one who wants to go, so I think you should just welcome her along.

I hope you enjoy mass. Happy Easter!