UK income - German Resident

I have a client, who is German and resides inGermany with his family.   Back in 2006 he worked in the UK at a subsidiary company of the German parent company from February 2006 until November 2006. He was paid from the German parent company.   The parent company treated him as an german employee.  From my point of view this is not correct. Since he physically worked in UK for more than 183 days the tax treaty between Germany and UK transfers the right of taxation from GER to UK for his income during his stay in UK. Therefore I am evaluating at the moment if it makes sense to write to the German tax authorities and ask for an exemption for the income related to his work in UK. According to German tax law to get the exemption it is necessary to prove that the exempted income has been taxed in the UK.  Unfortunately the client has not prepapred self assessment returns in the UK.  This will need to be done.  The client would have to file a tax return in UK as a non-resident for the salaries received during his stay in UK (Feb 06 - Nov 06). The advantages that I see for him are - personal allowances in both countries- lower tax-rate-zones in both countries. To check if my assumptions are correct I would like to ask you - if you agree with me, that the right of taxation for the income generated in UK belongs to the UK,- if he can file a tax return as a non-resident   I look forward to hearing from you in this matter. Please let me know, if you need any add'l information.