I strongly recommend you ask this question on the Software Quality Assurance & Testing beta site, or search for similar questions that have already been asked there. You will find a much greater depth of experience with using Selenium for record-and-replay QA.
Eric's answer is the experience of most QA Engineers who used record-and-playback tools: The resulting tests were brittle and difficult to maintain. A developer can use Page Objects with Selenium RC to make a far more maintainable UI test suite, and is a better option when it is available.
This isn't to say that Selenium IDE won't be useful for your tester; however, the usefulness (and the potential to hurt testing efforts rather than help) strongly depends on how fixed the UI is. You will get your best results with record-and-playback if the UI can be frozen early in the development cycle. You may need to teach the tester how to get new XPaths when the old XPaths are broken, and how to tell if the test is failing due to a UI change (because the XPath is no longer valid) or because of a functionality bug.
I do think using Selenium IDE is better than no automation at all for any project that has some generally-stable UI. Running hundreds of manual tests on stable UI every time that there is a minor change is a real time-sink. Just set expectations accordingly and realize that changing the UI will have a significantly bigger impact on the QA schedule once you start using record-and-replay automation for that UI. Your tester may want to pick-and-choose the areas s/he automates wisely.