I am one of those people who believes showers are for new mothers, and not for forcing your friends to furnish each and every child you have with large gifts. People usually do want to buy gifts for your children, and they will find a way to give them after the baby is born.
I don't think you should "encourage" friends or family to give ANY type of gift. Anyone who wants to buy a gift, and I'm sure friends and family will if they want to. Encouraging anyone to buy a gift is the very definition of greedy. Gifts should NEVER be encouraged.
Instead of a shower, I would talk with some of your mutual friends and do a girls pampering day or a girls day out. Maybe you treat her to lunch out and a pedicure. At lunch you can give her small gifts. It's not an official "shower" just an informal gathering of friends.
You could also ask friends to contribute and make a nice baby basket of goodies and have it sent to her house. Get a big basket or a laundry basket and then all of you buy stuff to put in the basket like diapers, toys, bibs, outfits, books, etc. If you get enough friends to contribute (and make sure you are letting them know that the amount of money they spend and what they contribute is strictly up to them) then you can make a nice big surprise gift!
Let her friends know it's a surprise, so that your pregnant friend doesn't come off feeling greedy.
Congrats to your friend!