@Baileysforchristmas - this will come across rather harshly, but you really need to firm yourself up more.
She is taking you for a complete mug - she is 21, not a vulnerable minority. And, as hard as it might seem, your son is actually worse. He is nearly 30, living at home and defending her actions. The living at home part is not necessarily the issue (completely understand how hard it is for his demographic and younger re getting on the property ownership ladder, it’s a shit show), but he is nearly 30 - that is way old enough to understand many other consequences/repercussions - and also to acknowledge how blooming supportive his parents have been in his life so far.
Yes, life and all it’s situations are not easy for them - and made even harder with the pandemic at the moment. But if a 30 and 21 year old can’t even remotely understand the POV of the people putting a roof over their heads then it is really time to cut the apron strings. You are, in the long run, not helping them - and you’re definitely not helping yourself either.
If you keep facilitating this behaviour, it will never end. I know I keep saying it, but he is nearly 30. Your job now is not to cushion him through adult life. He needs to create his own life and circumstances. Your “job” is to be there as counsel if he falls, provide guidance, encouragement, refuge if needed. But not to be there as a permanent flipping b&b.
Not keen on casting opinions on other women’s decisions, but the fact that she’s headed off there really doesn’t cast her in a good light. The fact that your son is ok with her going without even knowing the full name(s) of who she’s gone with speaks volumes too - total case of not wanting to rock the boat. This is NOT right - if the roles were reversed it would be red flags/LTB all round.
I really hope you don’t allow her back in the house - not just for the Covid issue, but because of her free-loading and blatant disregard for your wishes in relation to the health of your parents. I would keep those at the forefront of your reasons. I’m sure you probably would also add her probable manipulation of your son’s emotions - but keep that to one side. That’s not your “job” and that is where you could end up with a damaged relationship - he needs to figure that out for himself. In the meantime, the other two points are more than strong enough for your reasoning.
This obviously can’t be easy, especially after the last 9 months and with most of us wanting a happy, fun and safe few days over Christmas. Just remember that your views matter just as much, so don’t be swayed by guilt.