Many people have asked questions on this site regarding the rules for layovers, with no definitive answers. The rules are constantly in flux and it is very difficult to get a clear answer from government websites.
I would like to buy a plane ticket for my mother-in-law (who has a visa) from Ukraine to the US. Most available flights have at least one layover in Europe. My understanding is that EU countries (minus some special cases and US citizens) are currently banned from travel to the US.
If we buy a ticket with a layover in, say, Switzerland, can there be a problem? Is this allowed under current rules?