Hi, Rusty!
Our dogs and cats have always come away on holiday with us...we have rented a house or cottage for that purpose.
Our own pets do not seem to care where they are, so long as we are with them....
But to be on the safe side and make it extra familiar for them - particularly should we go out and leave them alone , for instance - I always leave familiar things about.
Such as a sweater of mine or my husband's, on which they might lie...perhaps leave it on chair or on a bed....
This gives the animal confidence that you are staying there...or should you go out, that you will return.
We left our cat indoors on his own in the holiday house in Swanage, while we and the dog went out, and the cat was perfectly happy.
Tell your daughter- in -law not to be too " tidy", but to leave familiar items about for her dog to see....and sniff!
And when he's poo'd or pee'd in the garden, don't be in too much of a hurry to clean up after him...leave the poo there for a day or so, so that he becomes used to his own smell in the "new" garden.
Maybe suggest that your daughter-in -law does all this, as soon as possible, if she isn't already doing something like that?
I hope this helps.