I’m guessing this means “he” is together with Dewi Lingo at “his” own place, not at Dewi Lingo’s home, right? How would you say the latter then (He is at Dewi Lingo’s place)? “Mae e gartre Dewi Lingo”?
Mae e gyda Dewi yn nhŷ Dewi
Mae ei gyda Dewi yng nghartre Dewi
Either of those would do to specify Dewi's place.