I may eventually redo the gate-keeping logic on this but in short...
SPOILERS AHEAD
.
.
.
.
You need to have a good relationship with Nick, then in Chapter 11 when Alex and Nick are talking in the park, Alex needs to ask Nick if he still cares about Ronan, then say "Ronan still has your heart." Later in the same chapter, when Nick is talking with Riley, select "I'm not sure" or "All yours, kid." That should unlock it.
Basically I was trying to avoid the player accidentally setting themselves up for an NTR type scenario.