The one year I received a 1099 while furloughed (2004), it was 10% of the full coach fare (I don't know if that is Y or what) for each leg. So for a two leg round trip it was something like $80 a leg, or $320 of imputed income. That's a lot for a furloughee with a wife and 2 children, multiply by 4. I think we ended up that year with $6,000.00 in imputed income on our 1099.
I didn't pay social security on it and was prepared for an audit but it hasn't come yet. I didn't because it isn't earned income.
So with only one person as a guest and travelling maybe 5 times a year it really wouldn't be that much, especially if they are able to take direct routes.
that's my 2 cents, hope it helps