Employee Taxes on the "Raw" salary are approx ~23% usually, that does not match your 5k. Are you talking about the yearly "income taxes"? But then your 45K were not "raw" in the French sense?
According to: http://www.salaire-brut-en-net.fr/ The after-employer-tax, he should get: 34650k
From which he still need to substract the income tax which is paid directly by employee. It is roughly a month of after tax salary -> 2880.
In total, I would say that a 45k gross income becomes 31k after tax.
Edit : I've updated it, thanks for spotting that one. I should have talked about "net" revenue when doing my calculations !