Thanks for your reply. I should have added that the reference his wife makes to him changing access due to work commitments is when he needs perhaps 3 times a year, travel abroad for about 4 days. He always has given her at least 7 days notice if his plans change, whereas she calls with less then 24 hr notice. She says that as the girls main carer that it is none of his business what happens prior to his pick up time at 6pm on a Friday and she insists they are returned promptly at 6pm on a SUnday - EXCEPT when it suits her.
Their 12 yr old daughter had the text sent from her phone, but the 12 yr old also saw it and has spoken of seeing other unpleasant tests her mother has sent her father. My partner has not risen to her insults as he believes it is best to 'keep your powder dry'. He is also dismayed at his wife's constant use of the children as conduit. Mediation has been suggested but he refuses to be in the same room as his wife. Their marriage collapsed due to her infidelity.