First and a bit off topic, Silver Creek, I admire your encyclopedic posts greatly, although this one sounds like a repost (and yes, have heard the complaints about the stomach pump, so much so I only use it sparingly myself).

flytyerinpa: Unfortunately, there is no way of knowing what your fishing partner is using, unless you can post an image of said fly. There are any number of types of food that are grey and found in stillwater, and okflyfisher along with NJTroutbum have hit the usual suspects (mayfly, chironimid, scud, cress bug) and I suppose one could add caddis to the list too (not sure if these are as prevalent in PA lakes as western stillwaters). Size 18 sounds a bit too small for callibaetis, but you never know and could just as easily be another mayfly. There are any number of patterns your could try from scud patterns to the Miracle Midge to the Muskrat Nymph, and beyond. You said he was fishing this with rising fish, but you did specify the type of rise form and if there was a hatch. Splashy rises usually means caddis. Less energetic but noticeable rises means mayfly either duns or emergers. Very light rises often denotes spinner takes. ScottP's recommendations are good ones because those patterns would cover all these situations. I might add to the list a WD40 in grey, which sort of looks like a mayfly, but is basically a midge pattern and is easy to tie i smaller sizes. You could also seine the water to see what's around, if you forego the stomach pump.

A final possible solution is to steal his fly box the next time he's not looking...