Hmm my friend know my own language and english so he want to learn 1 more language so which you would prefer him german or french? I think german that it is better.
Tough one. German is spoken in many of the world's top economies including obviously Germany itself. French however is spoken in half of Africa, Canada and some parts of Asia too. While most of these are at the bottom of the 3rd world countries list, they can still prove to be important markets in the future.
Myself I'd pick German mainly cause of bias, but it would be a good option to know them both. Also in the EU French seems to be accepted as 2nd official language after English for some reason. Probably cause many of the institutions are located in Brussels which is a mostly French speaking town.