We are hoping to allow a customer use of sharepoint designer for the purpose of creating workflows, is there any way to restrict the areas or functions they can perform within designer to try and prevent them from fiddling with master pages or anything else.
SharePoint Designer enforces the underlying permissions that are applied in the SharePoint site. If a user only has read access to the Master page gallery in a site then they won't be able to edit master pages. But if you are asking whether
a user who is a site collection administrator can be restricted to using SPD only to create workflows then the answer is no.
Paul Stork SharePoint Server
MVP Chief SharePoint Architect: Sharesquared
Twitter: Follow @pstork
Please remember to mark your question as "answered" if this solves your problem.