Ow,that's bad.
Hope he returns safe and sound.
I got a similar story of stupidness, but not within my relationship, but with a friend.
He got kicked out of his house by his wife and asked if he could stay at my place.
I told him he could, and explained the simple rules we have in our home:
Rule Nr 1: Take care you don't bother eachother in a negative way.
Rule Nr 2: Keep the doors locked at ALL times to make sure the dog does not walk out of the house.
My dog is kinda stupid and crosses the road whenever he likes, without paying notice to anything around him.
Next morning i wake up to someone banging on the front door.
It was one of the neighbours boys, telling me that my dog is wandering aimlessly through the block, and had been doing so since 5AM....

My friend left the door open and the dog had escaped when he went to work.
Mind you my friend had to look for another place to stay.
Hope your friend is still allowed to stay, though...