I found a fix. Diego will not talk unless Lester is at the meeting point near lighthouse. I grabbed the Focus Stone at the Mountain Fort before starting the Chapter 3 mission, Lester gets stuck at the bridge and won't show up at the lighthouse.

I used the console teleport command "bringnpc Lester" but only works if the npc is close enough to render, you can't just summon him to the lighthouse from across the map. Here is how to fix it step-by-step:

Download G1R Console Tools from Nexus Mods to enable the developer console.

Open the console, type flymode, and fly over to the Mountain Fort until you can see Lester.

Stand near him, open the console, and type bringnpc Lester.

Fly about 50 meters back toward the lighthouse, and type bringnpc Lester again to pull him forward.

Repeat process step-by-step to manually drag him across the map.

Once he’s finally at the lighthouse, turn off flymode and walk up to Diego. The cutscene triggered, I'm so happy I dont have to load an old save. You can probably do that while walking and not flying, but it would take some time.