Look at the CC&Rs. The HOA is responsible for the common areas. If one resident does not like what is being done with the landscaping, then she can make her issue known to the HOA. Then, depending on the rules and regulations, the HOA would deal with it according to the normal practices.
Just because one owner is not happy about the landscaping does not mean the HOA has to comply.
Next time, tell her that is what the landscape committee chose, and if she is not happy about it, she can petition to have it changed or get elected to the committee and work from within to change.