It's never good form for a woman to throw her own baby shower, but some ladies simply don't care for convention, and besides, they'd rather have things done their way. Whatever the case, it's always a mark of better etiquette on your part to arrive in good spirits and go along with the plan.
That being said, you don't have to feel totally robbed of your shower-planning privileges. As a close friend of the mom-to-be, you can take certain liberties when it comes to making sure she has a few nice surprises in store for her, no matter how well she thought she micro-managed the whole thing.
If you're the conspiring type, get all the guests in on a mutual surprise. This can take the form of a big, special gift that everyone chips in for, or perhaps even a guest appearance by her favorite local celebrity. You know her best, so shoot for the moon.
Your friend probably has the baby shower favors all covered, but you can show up with a few surprise edible items that even she wouldn't have thought to cater or prepare. Mini personalized baby shower favor pops will instantly brighten up any buffet table.