my son is grounded, his father has midweek visitation, he is currently out of town working, can he exercise his visitation and let him go to friends?

I'm not sure I can give you a conclusive answer based on the information you provided, but I can give you some preliminary thoughts. First, although it would be nice for his dad to respect the discipline you've put in place, he is not required to do so. Second, if your parenting plan has a "first right of refusal" (e.g. if dad isn't available during his parenting time, you have the first option to provide care for your child before any third-party is involved), then you may be able to prevent his dad from letting your son stay with a friend overnight. If your parenting plan does not specifically have a first right of refusal provision, then I'd need to know more about what the paperwork says before I can tell you whether you can keep your son or whether you have to let his dad direct where he goes during "his" time. Finally, although this doesn't sound like an issue, I just want to make sure I've clarified that even if your son is grounded, his dad would still be entitled to exercise his own time if he was available (in other words, you can't "ground" a child from the other parent, even if the other parent doesn't respect the terms of the discipline). I hope this is helpful!