There is no limit. If they are coming bakc as a contractor, they must be self employed in some capacity, so there can be no restriction, as it would a restrain of trade.
The tax man is the only one who may have issues. They must ensure they are truely self employed.
Thank you we were having a discussion about this yesterday and someone thought you had to be away 6 months , but we have a guy coming back after a couple of months
not illegal catso, just depends on what your current employment contracts says. I was once a 'Friday to Monday' - left my employment on a Fri, came back on Mon, same job, same desk, same everything, except I was freelance, and have never looked back.