For me, the request conveys something special, a piece to wear for brunches, open houses or informal dinners among friends.
She could, of course, wear a pretty dress, but in my city, that requires door-to-door transit and the assurance that she won't be seated near a drafty bay window.
Also, since party attire is not likely to be worn as much, she does not want to spend major bucks. Though a dress it-up-or-down piece is the holy grail, let us agree Marlene will not buy yet another pair of dark pants.
She also specified, "No red, no green, no snowmen." As if, girlfriend.
Hip and relaxed: a pieced Fair Isle sweater-jacket with a fur-trimmed portrait collar from Myco Anna. (The fur is recycled. Price, $215.) Not as heavy as it looks, with the collar opened and the neckline unzipped to the collarbone. They sell out of these every year and I can see why; the back is as merry as the front.
I dutifully trekked to the Montréal boutique (what a friend!) and can report the cut is normal, not skinny like the photo shows; it must be pinned on the mannequin.
Marlene could choose unusual pants and a simple top, of which she has "about four dozen".
The term "party pants" in my mind conjures clowns and balloon animals; however, I liked J. Crew's gold jacquard pants. (Sorry to say the gold ones are s/o in many sizes, but there are others on the site.) You'd think they might make you look bigger, but the idea, similar to an over-all print swimsuit, is to keep the eye moving, so they are surprisingly figure-friendly.
Marlene rejected them but said they got her mind open to something (as she put it) "kind of wild for me."
Glam in a quieter key means a subtler pattern like Boden's teal jacquard Bistro trouser, available in both a full and crop length (and up to size 22). These are lighter-weight cotton-poly blends, but washable. So wear tights if you live in the snow belt, and open the bubbly!
Price will depend on model, and ranges from about $65-$100 (plus shipping and applicable taxes.) The style is made in plains and patterns from wild to mild, but hey, live a little!
Some women will not wear this much embellishment, ever, and I respect that.
But you still want to look festive, not like you just nipped in from the dentist's. For that woman (and I am one except for three weeks of holiday season), I suggest Poetry UK's Neat Fitting Velvet Jacket, on sale for about $220. (I'm looking at the UK site, which serves Canada; there is also a US site.)
I see the jacket with Marlene's many dark trousers or a skirt, and masses of pearls, some on loan. The slate blue glows like moonlight on a Victorian roof, and the slate green is beautiful too, a colour that conjures a choir, holly leaf and verdigris candlesticks.
This is the top end of her budget but, since it is not as indelibly memorable as some of the other suggestions, she could wear it for a long time.
Whatever she likes, she has a day or two left to decide, as the holiday post is slower, and her parties begin in less than two weeks.