if the star is 10 ly from earth , and the speed at which alice travels to the star is 0.8c then the trip will take 12.5 earth years.

why would bob think it would take 25 years?

doesn't he own a calculator?

maybe he is thinking about how long it would take her to go
the 12.5 years to the star and the 12.5 years to return from
the star ... 12.5 + 12.5 = 25 earth years.

3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.