The sentence is a bit complicated because the so rendered phrase is apparently applicable to either faithful service, as Dan Bron has noted, or of the heart, which also works.
In addition to the sense of "given to" provided by others, rendered seems to borrow shades of meaning from other usages. I read it as metaphor - that Ms. Pross's unselfish heart, refined and purified by time, was such that all that remained was goodness. This sense of rendered comes from the rendering of fat to segregate the valuable from the corrupting. Something that is rendered is valuable, keeps well, and is the result of a time consuming process. This works pretty well in light of the preceding sentence. Rendering was done in the kitchen to make soap, and metaphorically, a rendered fraction carries an association of a clean and pure purpose.