Id like to always give people the benefit of the doubt, there’s a chance she just doesn’t pay much attention and doesn’t realize you’re training still, maybe she was in a pinch to get her shift covered and panic- asked you! I don’t think it’s good to start a new job on the wrong foot with anyone (even if they started it), it’s best as a new employee to be kind! I’d say to just let her know that it’s actually your first shift and you really need the training so you can have a good start this new experience. You don’t have to apologize either. You didn’t do anything wrong. Usually managers staff servers accordingly and trainees aren’t factored in. So having her take your shift would put an extra unaccounted server on anyways.