Hi BoF.
As other posters have said, JM scents are very light - she really doesn't do that heavy base that most perfumes have.
Chanel no 5 is a fresh, "classy" floral (the aldehyde) but it has a really quite sensual base, or heart, or whatever they call it.
I think you're right about Vintage Gardenia and the other poster who has recommended the Orange Blossom. Those two are the nearest in style to Chanel no 5. But ... they're not actually that near. Depends on what it is about the Chanel that she likes.
I think they're both a good bet, though I think the Orange Blossom is safer. It's a ladylike floral, that you can wear in the daytime BUT it has a teeny tiny hint of inner rock chick, which may pick up on the "mood" of Chanel no. 5.
All of JM's scents are, basically, very inoffensive. While women might not actually want to choose one as their true heart's expression, there are very few women who will actually HATE receiving one, and nearly all will actually quite like and wear anything they receive as a gift. Kind of like a white linen shirt, or actually, a T-shirt. So it's not that risky a gift. And perfume does look special when you give it.
(Inoffensive makes them sound awful - they're not, really, and I even have a favourite.)