Complex sentence: He says that he went into the building (presumably to meet with Ah-jung, having been in a relationship as his picture was on her phone), and he saw a distraught Ah-jung followed by Do-joon.
Split into simpler sentences: He says that he went into the building (presumably to meet with Ah-jung, having been in a relationship as his picture was on her phone). He watches a distraught Ah-jung followed by Do-joon.