Well,
1) The GMAT tests your knowledge of American English and some prep providers are careful to point out the differences & peculiarities between American English & Commonwealth English. It may be a good idea to at least get the Sentence Correction materials from those fiends of yours, after they have no use for them.
2) You may take advantage of many free materials here on GMATClub - tests, notes people shared, sample questions with discussions, etc - you can solve questions (whenever you have 5-10 minutes and you want a puzzle) and automatically have a log of it in your GMATClub "workbook" (a great tool) - this way you will have a great diagnostic overview when the time comes...
In all, my recommendation is simple - get materials form your friends and read the Sentence Correction and Math stuff for fun when you have time + become regular here at the Club - solve questions here, accumulate sense of the broader picture (people share amazing stories, note, insight), learn from our mistakes and (hopefully) successes =)
Buy the relevant, updated materials 6 months before the actual test day, then the hardcore prep begins =) If you hang around, you will need no advice on what to do when this time comes, you will be an expert =)