Parted/she wants to take them out of the uk? I pay cash sum every month.does have girls Second weekend's?

On Behalfe of very worried Son-Her name is on b/certifecate's Two daughters.He pays her a cash sum every week in her hand. She now has new fella? Wants to take them out of the UK, Any advice woud be welcome Please. He lived and surported her ( while she was in and out of job's more times than a yoyo goes up and down )for Fourteen years and all that time she was sleeping around? Poor son was so busy working hard he did no know. He is now living on his own and has his girl's aged three and seven every seconed weekend. Henidolises them and would be really broken hearted if they were to go abroad. And so would we as thier grand-parents. Actually it would be Austraila? I have advised him to get a lawyer as soon as possible.