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"He surprised her with a quick trip to Cabo [San Lucas] to ask her to marry him," a source tells the magazine.
Jay reportedly popped the question with the help of "a very big diamond," according to GossipCop, which has also confirmed the happy news.
The former "The Hills" and "Laguna Beach" star started dating Jay last fall. Earlier this year, she gushed to People, "I'm in love, and it's been great."
Cavallari's rep has not yet made an official comment.
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