Alexa - this is an awful dilemma for you but we all have to play our part here and if your granddaughter chooses to leave the house that's her choice. But she doesn't then have the choice to return. That's clear - irrespective of where she's been (whether she's virus free or otherwise). It really is up to your son to explain this to her. If he takes her back in he's enabling her arrogant attitude. I have had to have this conversation with several so-called 'sensible' people who just don't seem to grasp that by allowing people in and out of their homes they are putting themselves at risk. I can see how for people in a romantic relationship but who don't live together this must be an awful time, but it's no worse than close family members being separated, and there are many elderly people who don't have smart phones and who can't see their grandchildren who are probably the absolute light of their lives even through face-time or other platforms. Your son should be told to keep her in - or tell her she can't return - it's no use saying we wouldn't be told when we were young - the situation is totally different. We weren't growing up in a pandemic that threatens to totally destroy human beings.
I know some readers might thing this is all doom and gloom but a few weeks ago when I was urging people to socially distance lots of people were saying this was nothing to worry about - now the entire country is lockdown so it's time to stop concerning ourselves about whether we hurt someone's feelings or not, we need to step up to the plate. Your son must stop enabling his daughter and whilst he may not be so responsible to do that, it's your social duty to at least tell him. Likewise the boyfriend's parents are enabling their son to break the rules, and how comfortable are they with the idea of your granddaughter turning up and spending the night in their house - I am sure there will be no physical distance between the young couple. The sensible thing would be for one to move in with the other if the relationship is that strong - otherwise, both sets of parents need to be responsible adults and stick to the house rules imposed by the government. Good luck.