Assuming he's from the UK/ EU and he really doesn't want to be found, no I don't think he needs to 'register' in the same way a non-EU person would (work permit, green card etc). Depending on the kind of work he does, perhaps there's a professional body for his profession - journalist, designer, engineer, whatever that may be -- or a limited number of agencies finding work for people with his skills - you could search for them? If he's unskilled/ semi-skilled he might have registered with the Arbeitsamt Munchen (Tel. +(49)(89)5154.6017) or one of the 'Zeitarbeit' agencies that specialise in offering work on a daily vbasis.