I would not step in the middle unless you are positive that his actions are having a direct affect on your sons. Like lbug said, his R with them is his to screw up. You are there to support them and pick up the pieces when he screws up their R.....and he will...trust me.
Now, you grounded S16 from sleepovers in general or sleepovers with just certain kids? Was H aware of this? If so, then reminding him in a gentle way would be called for. Not in a confrontational way but in a, "Hey, I'm not entirely sure, but didn't we agree to restrict S16 from staying with so and so (or restrict him from sleeping over period..whatever the case may be)? I have this nagging memory of it but want to make sure so I stay consistent."
Let it fall back to him to pull that from his memory banks. You aren't accusing him of anything, you aren't badgering him, you are asking for his help (something men LOVE to do), and you are pulling him back into the parenting decisions without nagging at him.
T19 M15 S19 XH47 M43 bomb12/4/07 PA5/07 S12/26/07 D final 11/17/08 Back together with no defined R 05/2010 confused....to say the least!!!