To get back to the essence of the question, if all you want is to have an on-hand explanation to justify why you oppose a Ritual-Spontaneous hybrid, you might try this chain of logic.
From ArM5 81-85, the rules lay out how Hermetic Magic Works.
There are, within Hermetic Magic, three distinct types of spells: Spontaneous, Formulaic and Ritual. These three are delineated with their own headings very clearly, along with rules of how they are cast and their relative advantages and disadvantages.
Besides being explicit in this delineation, there are blatant seperations within the section, for example a section on "Non-Ritual Magic" which explains options available for "Spontaneous and Formulaic spells, but not with Ritual Magic" (ArM5, 83).
You might tell your troupe that, given such clear rules, if they propose that one type of spell might be capable of more, than they show you where and how it is possible.
You might also show them that there already is an explicit section on Ritualistic Spontaneous Magic called "Ceremonial Magic" which explains, explicitly, the extent to which Spontaneous Magic may mimic Ritual equivalents.
Unfortunately, while this does include adding Philosophy and Ars Lib scores in exchange for longer casting times, it does not include the use of vis for permanent, boundary or high-level effects.
As a rule, Spontaneous Magic is there to produce off the cuff, lower intensity effects than Formulaic Magic. That's one of the reasons that Wizard's Communion does not work with Ritual Spontaneous Magic: Spontaneous Ceremonial Magic is materially different from the very rigid Hermetic Ritual Magic.
Having said that, this would be a good candidate for a Hermetic breakthrough, especially through the Diedne branch of magic, if someone were willing to go all Tehran nuclear-physics lab on the order....
As a more practical solution for your Giant player, there's a CrCo25 (I think that's the magnitude) spell (ritual) which improves any wound by one magnitude. Perfect if you don't have access to lots of books on the different forms of ritual healing.