I agree with you that LVN/LPN are licensed and responsible for their own actions/errors. However, because legally "assessments" fall under the scope of the RN, and she is charge nurse, I'd say she has some responsibility for the assessment of the resident after the med errors. If there was an adverse effect as a result of the med error, and the charge RN didn't recognize it, or didn't act appropriately based on the post-med error assessment, then she could be involved.
As charge nurse, she also has responsibility in making sure that all incident reports, necessary paperwork and notifications have been made according to facility policy/regulations, so the error also affects her that way.
I'll also add that many, many RNs go all the way through school with no exposure to LTC, so I see how she might be surprised at how much LPNs are used in LTC and the amount of autonomy and even leadership positions they have. It is very overwhelming for a new grad to be in a charge position or the only RN in the facility at a given time.
Like you said, the LVN's competency is verified by her license and hopefully your SNF's nurse educator does some med pass competencies as well.