I think maybe the OP is hoping for some other reasons from Gransnet posters which she could use to explain to her daughter why she would feel uncomfortable with strangers in the house, when they come to discuss the refusal later on between them.
She had agreed to daughter and boyfriend staying whilst they were away, so the concern is over the two friends she does not know, and possibly that they might bring more friends too?
I think my reaction would be to ask to meet the two friends, and if they agree to come to a meal, then run through a few ground rules and expectations and see how that goes. Tell them you love your daughter and want her to have a lovely Christmas, but you need to rest easy about your home. No smoking in the house, no drugs, perhaps washing and making up the beds, cleaning bathrooms and hoovering, getting in fresh milk and bread for you on your return. Plus agreeing to a Skype call on Christmas day. If they don't come, then obviously you would not be happy.
It is a long time till Christmas, you don't want this hanging over you from now until then with your daughter, breakdown in family relationships causes far more worry and distress than a few days over Christmas.
But only you know your daughter. If she lives a hovel, then I can fully understand your reluctance!