Could the player cast this in the middle of combat? Sure! It's only a complexity 18 ritual; that's totally doable via thaumaturgy-with-the-speed-and-methods-of-evocation - if you're willing to take a good half-dozen points of sponsor debt (remember, The Dark Powers Are Always Willing To Help!)
You do realize that you're the one who determines whether or not he can take sponsor debt. Half a dozen? I'd likely have the sponsor ask "What's in it for me?" as that's a little above and beyond what's normally called for. Remember that on the same page as "the dark powers" sidebar (directly beneath it actually) is a sidebar that says this:
The GM should decide what level of debt
the sponsor is comfortable with before
calling in a marker. If a sponsored spellcaster
keeps racking up the debt but never gets
compelled, the debt has no teeth. Similarly,
there may be an upper bound of debt at
which point the sponsor cuts him off from
any more power. Every credit card has its
limits; but like a credit card company, a
sponsor is often willing to negotiate, if the
practitioner can figure out how to make an
appropriate petition.
As for the OP, I figure it's pretty well wrapped up, but I'd throw my word in there too (since I'm already posting). It's totally RAW for the aforementioned reasons.
