Most airlines will allow a carry-on dog, In an approved carrier,
under the seat. PERIOD. This is something from the size of a chihuahua to a teacup poodle. Industry standard is 9 inches tall, but can be as low as 6 inches. The animal cannot come out of the container.
Some airlines accept cargo pets. Waits are LONG, before boarding, during layover and upon arrival. Typically the animal goes in the box hours in ADVANCE of loading the cargo. During my layover in Frankfurt, the dog was supposedly watered and exercised.
I was not allowed to contact or visualize the dog during layover.
Upon arrival in
Portugal, it took them well over half an hour to produce the animal.
Clearance with the State vet was a chore in the US. A certificate from my vet was required before driving to another state to visit the REGIONAL "State vet" since the US is apparently no longer capable of providing one in EACH state.
Your friends not only need to dot their I's and cross their T's, they need to interrogate everyone in advance.
KLM, for example, has a pet policy they proudly announce, once you can find it. They
fail to point out that it is only available on certain flights at certain airports...why?...because as many airlines do, they book you on their airline, but you FLY on some other companies plane, who DOES NOT accept cargo pets.
Serious detective
work advised, and they are wise to fly based on the temperature unless they want to risk dead dogs.