You can try your best to help them come back to praying, ask them if there is anything in particular that is making them stray away from Islam. If it's someone dear to you, then i'd say to not give up on them and always pray they come back but don't be too overbearing. That can lead to them straying away further.
@VixR I've known close people to me who have had their doubts (I'm kind of dealing with one person rn) and it's always best to give them their space but have an open discussion from time to time.
@Luciddreamer seeing as you've had your moments before, i'd expect you to sympathise and approach vixr in a better manner if you really did care about her being a gaal. I don't agree with some of the things she says but it's like people on here pick and chose which gaal to be annoyed with.