Thanks for providing the link to the launch page. I've looked the scripts over and ran some tests, and I have been unable to spot any problems that would explain the "white screen" issue experienced by your participant. I don't think the browser used (Safari) would make a difference. Inquisit Web does not technically run "in" your browser -- the browser is merely the delivery channel for the Inquisit "engine" (the code that actually executes a script on your computer), the scripts and their associated files (images, etc.). The engine, however, comes packaged in a number of different ways to accommodate for different systems' and/or browsers' features, e.g. packaged as a Java application, as a browser "plugin", or as a browser-independent executable (the "Web Player").
If the participant in question, for example, used the Java method (the web log entries will tell your what variant of the engine was used), it may be worthwhile to have her/him try a different launch method (via the "Having trouble starting?" link on the launch page). Trying a different browser would have a similar effect in many cases, because not all browsers support the same launch methods (for example, under Chrome only the Web Player would be available, because Chrome no longer supports certain types of plugins, which rules out Java and the Inquisit Web NPAPI plugin).
Finally, based on the reported "white screen", it *could* be that the respective script somehow got corrupted when it was downloaded to the participant's computer. A white screen is what you would see when trying to run a script that basically does nothing, i.e., has no proper <expt>, <block>'s, <trials> and stimuli defined. Admittedly, though, this is pure speculation on my part. The chances of this happening are extremely low. If it is the case, however, then clearing the system's browser cache and temporary files prior to trying to start the experiment again should lead to "fresh" copies of the files being downloaded.
Hope this helps.