Hi, all! A little late to the party, but you've mentioned the same ones I'd have mentioned.
The Tradition figures are both still available at Tradition's website, by the way.
Rylit also produces a couple of figures of Frederick in 54mm; an email to Frau Litterscheidt via her website will return ordering info.
Shannon, is your friend looking for a painted figure, or a kit that he would paint for his friend? Ulrich Puchala's Frederick figures are much better than the Tradition figures, in terms of detail, but of course, someone would have to paint the figure first. If your friend is willing to go up in scale, Ulrich started sculpting 100mm figures a couple of years ago, and he has some figures of Frederick in that line, too.
The only other figures I can think of are out of print, like the old Stadden 54mm figures of Frederick, from the custom figure range, the figure by Old Guard, depicting Frederick walking with his greyhounds, the Franklin Mint figure, or the one by Archiv-Verlag in Germany.
Prost!
Brad