How long can a resident parent (UK) take child abroad on holiday without consent of ex?

My GF wants to take her daugher on holiday for three weeks - she is the resident parent, but her ex husband is threatening to go to court to stop her from taking her on holiday - can he do this? She's set to travel next week, and she informed him about this previously - but he's threatening this to force her to concede on some negotiations over separate financial settlements.