Excellent advise. The most difficult wedding cake I've designed so far has been for my dear friend who insisted the wedding had no theme and I should do whatever I want, because "it will be beautiful no matter what". I'm sorry dear, but that's not helpful. Every wedding has a theme/feeling, even if its more casual without a lot of "matchy matchy" elements or a specific colour palette.
I started by showing her a variety of cakes from the internet to guide our discussion, but I feel now that wasn't the right approach because it didn't really tell me much about what she liked about the example she chose. It took a lot more prodding about her dress, flowers, invitation, etc for a theme to coalesce.
My biggest problem was colours - all she told was "colourful" - not helpful! Primary colours? Slighly pale colours? Muted colours? There are many definitions of colourful. What saved me in the end was I asked her about her flowers and she showed me a photo of beautiful bright autumn flowers, slightly muted - sunflower yellow, burgundy, soft green and dark purple.
All this to say, it was a huge learning curve and I'd be interested to hear what other types of questions cake designers ask when the couple don't know what they want.
You sound like a great friend! I would love to see pictures of the Autumn wedding cake you made her.
If the client has shown me pictures of cakes they like, my first question to them is "what is it you like about this cake?" And then point out cake shape, colors, piping, etc. if they have trouble defining what it is they like. If there are several photos I do an "overall" look at them to find what's common about them. That's usually the key element the client likes and my starting point.