I was left wondering how memorable and unusual this symbol is if someone who (probably) doesn't have a copy of a love letter between the writer and her husband could get an exact copy of it done from memory. My guess is that it's not that off-the-wall weird or detailed.
I was thinking something like a yin-yang with hearts or a popular character with some symbolic attribute changed.
And if my assumption is right, then I don't think Kathie was that wrong to get it on her body, though I do think she ought to have mentioned it first! Getting a symbol on your body forever that you know symbolizes something significant to your close friend seems like a weird thing to do, then unveil at that friend's wedding.
FWIW, an acquaintance of mine had a unique logo (in a tattoo-like style) drawn up for her small business, and it's showing up EVERYWHERE in the U.S. on people's arms, legs, backsides, etc. as a tattoo, sometimes with her business name on it. If she finds out, she lets people know to give the original artist credit as some small token for the work she put into it (though many resist and have treated her very poorly).
I'd think that Kathie saying something like, "I got this to honor the love of my best friend and her husband, who drew it. Isn't he talented?" is about all the friend can hope for at this point.