With someone to fight against, Tally shows that she can actually be a good and loyal friend. This shows that she understands how important it is to respect Shay’s choices, even if they don’t make a lot of sense to Tally. However, Dr. Cable’s threat means that Tally is going to have to weigh her own desires against Shay’s. Given that Tally’s world isn’t one that promotes selflessness (she mentions at several points that pretties get whatever they want), it’s likely that Tally will also behave selfishly.