If the right of way is across her property, she has the right to dictate its appearance to some extent. What she doesn't have the right to do is possess it exclusively and bar your passage over it (such as impeding it with a gate, boulders or bollards). She probably doesn't have a right to preclude you from improving it or maintaining it as a functional road (such a paving a dirt or gravel road or installing drainage or swales, or widening it to the width in a deed or to a width adequate for two way traffic or emergency vehicle access).
In your question, however, the issues seems to revolve around aesthetics rather than function, and to these issues, I believe that the landowner who owns the fee title burdened only by allowing you a reasonable easement to cross the property can object to aesthetic or non functional improvements across her property, such as white rocks rather than gravel (YM "crusher run").
It's touchy, though and I would recommend consultation in the future before just doing something without discussion and consent. Tough question, but it seems more an aesthetic issue than functional if you could have used either grey crusher run gravel (pea sized gravel pieces, like used in playgrounds) rather than this white cobble which seems to be more conspicuous and is what the neighbor may be upset with.
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If it exists look at the language of the easement/right-of-way. This may provide guidance on maintenance issues and disputre resolution. If the document is silent on maintenance, then it may be difficult to determine anyone's rights in the matter. If it escalates consider mediation and/or hiring an attorney.