Send An "Accidental" Text Message

Oops! Wrong person...

Something like "OMG I'm really not supposed to tell you this, but I just found out something MAJOR and I need to spill the beans. Promise not to tell anyone?"

Quickly follow that text up with, "Sorry, wrong person!" Your gossip-addicted friend will surely text you back, demanding to know your secret, but instead of filling them in, you can string them along all day before revealing that the major development is how gullible they are.