Jang Ye-won is speaking informally to Seo Jang-hoon, who is 16 years older

They both appeared together on "Secret Member" (Biseojin).
There were times when scenes of them chatting privately while riding in a car together were aired without being edited,
and I was surprised to see Jang Ye-won speaking informally to Seo Jang-hoon.
Even if they are close, she is 16 years younger and much more junior in her career.
With that much of an age gap, isn't it difficult to drop honorifics?