eBay and Craigslist finds

My girlfriend is after a cheap racing bike (with gears) for a triathlon in August. She normally rides fixed but they ain't allowed in the race.

Anyone got something for sale (or Ebay suggestions)? She wants to spend £100 max. Frame size around 21 inches or 52 cm.

Maybe just a single speed freewheel or a rear wheel?
Seems to be the easiest and cheapest solution if fixed gear is not allowed.
Or is single speed not allowed ?
Strange rules...