How does the reader know that Mark's girlfriend, Natasha, feels threatened by Bridget?

At both the book-release party and the Tarts and Vicars party, Natasha takes every opportunity to make Bridget look bad in front of Mark. Her behavior suggests that she sees Bridget as a threat to her relationship with Mark. Natasha's remarks are aimed at forcing Mark to compare the two women in the hopes of Bridget coming out looking bad. In reality, her behavior ends up making herself look bad.