Look down to my phone in chat and tell everyone I saw Phil.
I'd engage him in polite conversation, then probably mention casually that I'm a member of Kiwifarms at some point. I wouldn't actively be rude or aggressive with him, I'd just be curious about his general every-day activities and how he acts when not shitting up Facebook. I'd consider myself as having much the same attitude as a naturalist; observe passively, and allow the target to act naturally. Has Phil actually met a member of the farms before? By this I mean one that joined prior to meeting him, rather than afterwards to share their experiences.