If they keep it up, I'd check the law out.
I know here it is against the law for a dog to be off leash unless they are in a private fenced-in yard or a designated off-leash park. (Which just made me wonder what the law is for these wireless fences that have become so popular around here?

)
I know in my last neighborhood (there were condos that didn't allow fences on the next street over) the dog catcher would make regular rounds and take any dogs not supervised and ticket the owners of dogs not on leash.
If the dogs is on a leash while messing in your friend's lawn - well that is just so wrong that they deserve whatever creative punishment that YTers come up with!