I have flown with guinea pigs as hand luggage (this was a few years ago though when such things were allowed - it is much harder these days).
They coped with the trip (a 90 min flight) absolutely fine.
But as Free Ranger has already said, your friend might need to reconsider.
Firstly these days it is extremely difficult to find airlines that will fly piggies in the cabin - if they accept them at all it is usually in the hold (a part that is temp controlled). Getting space here needs to be booked far in advance, and they may end up sharing the space with cats and dogs, which I imagine could be stressful. This is simply because cats and dogs are more commonly flown. You also always need a Plan B, as any booked animals may be bumped from their flight if a cargo that has greater priority is booked at the last minute - it's rare but can happen.
Not all planes offer a temp controlled space for animals so she would need to choose her flights with this in mind.
Secondly can she be sure of access to reliable vet care and all the necessary supplies including things like decent hay year round.
I know it sounds like a great idea, but the reality of moving an exotic pet like a guinea pig to a new country can be a lot more problematic than you think.
Has she been there and seen where they will live, and been able to verify that she can find everything she needs, and also have a safe space for the piggies upon arrival?
