It started during the Battle For Manhattan - Demigods were expendable to the Titan Army, so they didn't bother having healers. When a Titan Army demigod got injured any more than they could care for themselves, they were left for dead.
In the battle, they'd lay injured and see an orange camp shirt weaving between the fighting like a flag, heading straight towards them, and they'd presume they were about to meet their end. Instead, a blond-haired twelve-year-old would scurry to their side, weighed down by first aid kits and archery equipment - one of the few surviving members of the Apollo cabin- and would start tending to them and helping them back to the safety of Olympus' forces' makeshift base.
It had been a rough week for Will- Cabin Seven had been one of the most populated cabins at CHB prior to the battle. Within the span of just a few days, they had been reduced to just a mere three of the youngest siblings. That was enough to put a damper on Will's mood, and then on top of that, he was suddenly responsible for almost solely taking care of dozens of injured half-bloods.
He treats ex-Titan Army and CHB demigods alike with the same stern seriousness. No snide comments, no judgements, just curt directions and orders so he can patch them up as best as he can. He's not rudely impersonal either, though. To the kids he doesn't know, he'll ask them questions as he works to distract them from the pain- their name, who their godly parent is, where they're from, something they like. He treats them as equals to his fellow campers. The Titan Army kids appreciate it- it's a kindness they're not used to.
He saves their lives. In return, they stick to him like glue.
Lou Ellen is one of the first Titan Army kids to approach Will after the fighting is over, as CHB rounds up the demigods left and starts sorting them out so they can join the camp. The children of Hecate had been a close-knit unit in the Titan Army. They trusted each other. Lou Ellen trusted Will - he had been healing her (and bantering back and forth a bit) when Nico had arrived with reinforcements- so the Hecate kids trusted him too. They tell the others, who tell others, each adding their own stories to back up the claims. Within the first days, the ex-Titan Army demigods are at camp, they've all deemed Will a safe haven.
Lou Ellen hovers close by Will's side whenever she can for the next few days, not always directly interacting with him but present nonetheless. Most CHB kids had lost someone close to them in the fighting, many to Titan Army members. Even Will had- almost especially Will, nearly his entire cabin wiped out But Will was one of the few campers who didn't seem wary of the ex-Titan Army kids. He spoke to them casually, befriending them quickly. His calming aura helps- They're all nervous, in a new place with new people whom they had been at war with only a few weeks prior, who are clearly equally uncomfortable around them, but Will's aura settles their nerves. They act almost like bodyguards to him, willing to help with any tasks he needs assistance with, or just standing watch around him and chatting.
Finally, when Will can no longer stand the two groups self-segregating, he starts playing matchmaker. He's already well acquainted with just about everyone at CHB, particularly from having to play doctor for them all so frequently. Having befriended just about every ex-Titan Army kid as well, he starts making introductions. He connects kids with aligning interests, drops names, suggests topics to discuss, and even purposefully pulls a few half-bloods into conversation with him and then ducks out once they start carrying it on their own. Whatever he can do to get the two groups to start talking with one another. Slowly, camp unifies.
(And the ex-Titan Army kids don't forget- many of them practically vowed a life debt to Will for saving them. Even over a year after the fact, Will still isn't quite sure what to do about that whole situation.)