None of this really makes sense OP for the resins you say but also..
Umm, It would have helped if you'd explained lockdown rules, just a mention ... no one is assuming, we cannot cater for infinite variations or lockdown that may be in your area. If U.K., and wales ... be proud and mention it.
But you are right , if she (gf me) has withdrawn her DD from school due to her concerns about covid19 risks at their school at this time, then why would she allow allow your DS to visit her DD at their home, where presumably he has been at school each day/. It's Illogical
She is manipulating your DS. I would suggest at he doesn't visit if she is choosing to self isolate her DD at home. "One can't be too careful for GF'S sake"
And she - GF DM - can keep her views to herself
I have a particular place in hell for Interfering unrelated parents.
DS goes to school or you get fined , it's not rocket science that DS cannot understand, GF's DM can fight her battle and take her own fines.
DS lives with you and is your teen. I don't know how you do that, but I would start with "if GF is isolating at home you cannot visit her because otherwise it makes. Mockery and also your need to isolate for 14 days first before you could visit her according to her logic" ...
And even if you DS, don't go to school when I tell you to, you can pay fines from your Xmas budget, I will put headteacher on the phone to you DS so you can explain (& try to relay GFs mums craziness yourself) and you can take it in chin DS if / or when we get CSD referral about your delinquency that school will inevitably do where YOU explain Your actions Ds and GFs mum will not be involved to 'rescue you' . You go to school and stop this nonsense as it is illegal.
I wouldn't brook cray arguments and I'm
Shielding group ... my DCs go to
School. Half of my youngest's year group were sent home to self isolate for 14 days . Mine stayed in school as not in that particular bubble . No one fights snd wins against the system. 