Beginner Bikes Help

I’m looking for an inexpensive road bike (< $400) for a friend doing her first tri. She will probably need a 54 cm or 52 cm bike. What other places would you look at to find a bike in this price range? I have already tried Craigslist. What models would you search for on ebay? Is she better off just using some mountain bike till she can spend more money? The bike leg is only 8 miles. For the next few years I can’t see her doing anything beyond a sprint. Please let me know if you have something you are no longer using and willing to get rid for a fair price. Thanks in advance for the help.