Up to you, Dom. Doc could work for Lucy, or stop by the clinic when he's in town, or work keeping the night-shift ghoul guards (ghouls guard the gate at night, humans guard it during the day) healthy, or whatever other excuse you want. Independence is as welcoming of ghouls as any settlement can be, though Doc's admittedly hard to get along with for long periods. They like his caps, though.
Also, I will echo Dom's "What the hell", though I'm going to roll with it for now.