Our "modern" society in these regulations assume the average person has one single full time employer

The employee seeking extra pay by having a second employment does make for an interesting position but not one you can have any control over.

Just because an employee is not physically at work does not necessarily mean they are resting BUT had you remained unaware of their second job it is likely you would have assumed they were.

Options for gaining control could be to pay the living wage (or better), provide opportunities to work controlled overtime or start a capability dismissal when they arrive unfit for work due to tiredness.

In the menatime lets see if our elected leaders can get to grips with the foibles that are the gig economy 

Looking at the regs (Reg 1) the employer is only responsible for “working time” which the time he is “at his employer’s disposal and carrying out his activity or duties”. The regs specifically exclude things like time travelling to work.  If an employee decides to do another job, that is their prerogative. If you feel that the employee is not fit for work due to fatigue then you should mention that.  HSE guidance says “The legal duty is on employers to manage risks from fatigue, irrespective of any individual’s willingness to work extra hours or preference for certain shift patterns for social reasons. Compliance with the Working Time Regulations alone is insufficient to manage the risks of fatigue.”  Unfortunately the guidance focuses on shift patterns and as has been said does not mention working more than one job.  I suppose in theory you could tell the employee either give up the weekend work or find another job if you really feel that their levels of fatigue put him (and the business) at risk.  In reality I can’t see this happening.

That they like or need the 2nd job is not really your issue.  The OP is really, in a sceanrio situation is askng 2 quite seperate questions.

1. How do they stand under WTD and that seems fairly well answerd by the extract from the Regs - doesn;t affaect you

The 2nd is that they are, to the OPs view, not having suffu=icient rest and are doing other work.  For all we know they do it as a charity, for the social interaction instead or or as well as the money. As some have said, had you not been told about it, would you have picked the worker out for failing or tending to be tired/ fatigued beyond the other employees.  Would the OP scenario be as concerned if the employee habiyually went walking or the cinema every weekend? I really dislike this thing about fatigue and tiredness, its usually an immeasurable concept.  Given half the chance I'd have a doze in the afternoon, is it because I'm tired or that I just like to have a doze? Would it realy make a difference to my work or not?   Its good that the OP considers the employee but not so sure they have a duty or responsibility to them if their behaviour at work is acceptable. Fatigue, to my mind, is a fair way past a bit of a late night 

I agree with many of my fellow forum users.

If the employee's performance isn't suffering then unfortunately in reality there is very little that you may be able to do - especially as the employee's extra-curricular activities are in their own time.  Would you act the same if the individual was playing sports every night or part of an amateur dramatics group (I know from experience that rehearsing for shows is extremely tiring!).

However, I would arrange for someone (you or line manager) to meet with the employee and have a discussion with them and find out some more details (in my experience getting HR involved too early makes a mountain out of a molehill).  Whilst with the information we have it would appear that the second job is a contributory or causal factor in their tiredness, it could be a number of things combined.  For example - I have an underactive thyroid so I tire more easily (despite the medication) and can sometimes find myself yawning despite my mind not feeling tired.

One thing that you can do, if business needs allow, is move towards agile working - so help the employee manage their work life balance by giving them the option to work alternative hours.  My organisation champions agile working by not giving set daily hours.  Instead, contracts state the number of hours to be worked in a week together with core hours (e.g. 9.30 - 2.30) that employees must be in the office or at work, and then outside of core hours, completion of weekly hours is flexible based on individual needs as long as we don't go above a certain number of hours a day.