Why do you want to loop inside a workflow (usually in more than 90% of the cases, there is a better way to achieve the same logic without loops).
Loops in workflows are known to be a potential performance hazard since you risk ending up in an infinite loop. Generally I would advise against that …
On a side note, any reason they customer is still on 6.8.1? That is a very old version with tons of performance issues and bugs fixed in later versions. I would definitely recommend getting them on the latest version asap.
May I ask the following: