Slightly belated, but here are my ideas:
1. Use Track Changes, as Beetee suggested. It works the same way as just changing, but it shows her where her mistakes were (and how you fixed them).
2. If you don't have Track Changes available, then use the comment feature to point out mistakes for the first part. (I usually do it for the first page of an essay, but you could also do paragraph.) After that, just correct them without commenting.
3. Either way, offer some general comments at the end to help her focus her revisions in the future. What errors is she making over and over? For instance, maybe she keeps shifting verb tenses or has a tendency to use too many commas. Let her know so she knows what to look for when she does her own proofreading. You can probably just be direct and simple for most mistakes. ("Watch out for run-on sentences.") Use examples and more detail if it is something more obscure, like using the wrong word. ("In most cases, 'affect' is the verb and 'effect' is the noun. You got them mixed up twice, once on page 2 and once on page .")