I agree that (whatever the colour, whatever the relationship) you should address this politely but firmly asap with a neighbour face-to-face chat.
My experience (7 fence negotiations in 6 years, don’t ask) is that it’s a slippery slope if you don’t address it early. Whether they are oblivious of the law/common courtesy, or CFs who do it on purpose to prove they are king. Either way, there’ll be more small transgressions once they get a taste for it. It’ll add up to real stress over the years.
Wish it weren’t true, but I’ve seen it too many times with too wide a variety of personalities/ages/etc. With great friends, complete strangers, and everyone in between. Nowadays I address it upfront, with a big smile and supporting documentation. I lay out what everyone’s rights are, I let them know my preferences and ask them theirs. I take notes. I keep in touch with any changes (mine & theirs) and talk budgets, tradespeople and cosmetic stuff (like paint) early on. Even then, I’m always always surprised by how much emotion is evident around fences. And how riled perfectly (otherwise) reasonable folk get over fences, and not getting every tiny thing their own way.
I’ve been sworn at in front my kids, suffered midnight fence raids where they changed it against the law, had mature boundary trees butchered, and had neighbours try to bribe contractors to build things ‘their way’ (and against the law). They can’t seem to understand that a ‘win’ today might mean years of bad feeling, with people they live metres away from. They do realise it afterwards, in most cases. Some just escalate forever. You know, the types who’ll enjoy whacking you over the head with any olive branch you offer?
Sorry for the long response- short version is go and talk to her. Be smiley but firm. Approach it as though it was an innocent mistake. Clarify what you’ll be doing now to your fence, who’s paying and what you expect in the future. End with ‘good neighbour relations are important to us, and to you too, we know’. Good luck!