Heather is about 5'9", but she's a skinny minnie! With a tiny waist and a lean frame with legs that go on forever, she's definitely a stylist's dream! Even after a baby, to us, her friends, she looks exactly the same as she did pre-preggers (truth!).
So here's a suggested outfit for my Heathum, just a concept of two styles of the same look that I drew:


Starting with the jeans:

National Jean Company - A Pocket Fit Flare Jean in Vintage Allston. $245.00 (but 20% off with the code "grechen")
For the cool, casual, "mom on the go," I would suggest these Seven Fit 'n Flare jeans. Lucky for Heather, who doesn't have a corporate dress code to abide by, she can go with a few stylish tears in her jeans.
My take on tears: I don't like overboard tears for casual wear. Maybe if you're trying to make a statement at a fancy dinner or a casual club, I would wear skinny jeans with crazy tears (best when they're NOT in style - lends more to the rebel vibe :). But little tears like this I think will never go out of style when paired with equally casual top and accessory pieces.
Another thing about these jeans - they look best on women with long legs. On me, the knee on a pair like this would hit my shin! I have a pair of dark wash Seven jeans that are similar, with small knee tears, that I love to run errands in, but I had to hem them. For Heather's slim legs, jeans that "fit" at the thigh and "flare" towards the ankle are best so she doesn't look like she's drowning in the jeans.
Another jean for a more polished look, and a brand I personally ADORE: J Brand.

J Brand - 722 Lightweight Lovestory Bell Bottom ($188), 20% off with "grechen"
Worn by Jessica Alba, Rhianna, etc. etc., these are definitely one of the "celebrity jeans." But seriously, I know why. It's because they fit like a dream!
And to top off the outfit, who styles casual-comfy-chic better than James Perse? Either of these will do:


Not the most economical choices... but we're still in fantasy land aren't we? Brace yourself:
James Perse Oversized Cotton Shirt, $195, net-a-porter.com. James Perse Sheer Cotton Shirt, $195, www.saksfifthavenue.com.
My philosophy on basics: These prices might seem ridiculous, and they probably are for most so I'm not saying buy these specific pieces, but I would definitely call this shirt a basic. And, contrary to intuition, I would invest MORE on basics than on trend pieces. Quality basics go a reeally long way when it comes to both surviving seasons and looking polished.
The flare of the jeans with these tunic styles will really create curves and movement for the very tall and thin. You can even cinch it with a belt if you're feeling fancy. Or throw it on and roll up your sleeves before you run errands. The loose fit keeps it casual and effortless, but you will look great! Sheer is very in right now, and pairing the white shirt with a camisole underneath, experiment with the color :), or even wearing a nude bra and daring to wear it with nothing under (lingerie-inspired) along with a blazer would be SO hot, such as:

Elizabeth and James, Double Breasted Blazer, $395.00, shopbop.com.
Personally, I think if you found a brown jacket, that might go even better. This is definitely an "in your dreams" pricetag, as is everything else in this post, but if you're looking for something different but casual still, this might fit the bill:

Lela Rose Cashmere Safari Jacket, $995, shopbop.com.
The details here are so unique! From the shoulder panels to the cinch-me or don't cinch-me belt, to the collar that can be worn up or down to the hint of metal in the button and belt details... And this jacket will contrast well with the simplified elements of the rest of your outfit. You can throw this on over just about anything and look very sophisticated. This is definitely an investment piece, and the idea here isn't to buy these specific products, but that classics/quality basics + one investment piece = very polished.
Since Heather is tall, she doesn't wear heels very often. I would suggest any of these very cute flats (to go with her Coach baby bag!):
Coach Skyla Flat, $158.00, coach.com. Coach Honor Flat, $158.00, coach.com.
BUT... I will say - wearing heels with these flare jeans will definitely make you not just very tall, but very hot:

Christian Louboutins, Round-Toe Pump, $625, Neiman Marcus
Of course Love Quotes, go with everything - Love Quotes Linen/Viscose, $79, Singer22.com

But SERIOUSLY - red scarf with the red shoes, white shirt, either of the jeans, plus blazer? YESSS! :)
So, these are of course not practical suggestions due to the prices. But it's fun to look, isn't it! And hopefully it gave you a style idea or two that maybe you can find in a store at a more reasonable price :)