Yes, when a modulation is in progress, accidentals from the new (original, in this case) tonality are introduced (or reset).
We're going back to G major, and restoring the natural C is pretty normal in order to anticipate the "new" harmony, as it also leads to a better transition to the third, due to the chromatic interval.
I'd like to extend my answer, considering the one given by Tim.
I agree that it's not uncommon to have some doubts about written music (errors in the edition, tradition or performance practice, and even personal taste), and there is always room to some amount of interpretation (even extended, including note modification): we must always keep in mind the composition style and period as a context.
We should always keep in mind that, even in the "classical" period, interpretation also extended to customization of music to the musician needs, capabilities and taste, just like nowadays performing. Think about the changes in tonality of some well known Arias: even the composers were well aware about that.
In this specific case, which I'd say it's a "strict" classical form, there should be almost no doubt about the natural C, most importantly due to the presence of the chromatism at the end of the last bar in the modulation.
A classical modulation is usually done through two fundamental steps: introduction of the new tonality by destabilizing the previous one (normally by using the new accidentals) and confirmation (with a strong cadence); both those steps are important and must have enough "consistent importance" in order to have a smooth and correctly declared modulation. In this case, the new tonality is introduced by restoring the natural C, and keeping the C# - until that point - makes the modulation very short (almost unexisting) and somehow destabilizing; it wouldn't make a lot of sense to have a C# even at the beginning of the last bar and "clearing" it just before the end, considering the composition period.
Do note that, as always, these are no absolute rules, and harmonic and phrase contexts should also be always kept in mind.
For example, a continuous alternation of C# and natural C could be used, because the composer consciusly wanted to create an ambiguity for artistic purposes; or that alteration could be kept and even highlighted until the end, in order to create more harmonic tension: think about the infamous E-D# of Für Elise.