Do you have any parenting orders in place? I ask merely because this is something that would come under extra curricular activities for the child which would in most cases facilitate you attending to watch even if it does not fall on your time as shared parental responsibilities. Which can be both a blessing and a curse.
I dont want to make too much comment on the rest, honestly she is an adult and while you know him, you don't know her or their relationship. She has the ability to say no at any point in time, just like you do (and obviously in the end did). Concentrate on the important stuff, your children, not her and his relationship. While its admirable that you are concerned, its not really your place anymore. So long as the kids are well cared for then don't sweat the small stuff.
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