You need to speak to an immigration lawyer. I’m sure pandemics is considered legitimate reason or health issues, but have not seen guidelines by USCIS for someone who stayed outside more than 6 moths. It looks like case by case basis.Few airlines are doing chartered flights to the US and cost is 2000-2400 USD. As pyackman mentioned if I were you, I would ask mom to fly to the US, but hold any travel if she is considered high risk. No GC is worth a life