I can't tell, either; as a late 19th century cranky Scot, he's both likely to have thought "as if Fritz, hero and defender of Protestants everywhere in Europe, would!" and "of course Fritz, masterful tactician and faker if needs must, would, you naive Son Of Muller!"

But yeah, I'm with you: he would have if he could have. Meanwhile, here's how I imagine things went down in Potsdam:

Grumpkow: Your Majesty, the crown prince is now very devout, praying with Pastor Muller.

FW: Good.

G: He's sworn off the English marriage project.

FW: He'd better.

G:...and wants to marry an Austrian arch duchess, convert to Catholicism and move to Vienna.

FW: WTF?!!!!!

G: Just a suggestion, maybe allow him to play the flute again? Just as an alternative to Catholicism, of course.

Meanwhile, in a dispatch to Vienna:

Seckendorff to Prince Eugene: Things are still bonkers here, crown prince just proposed to arch duchess, plz ignore!

Eugene to Seckendorff: WTF????