Visa requirements for one-day transit thu Germany

My friend (Indian, currently in UK) has a Schengen visa issued by French embassy. Is it possible for him to change his travel plan through Munich (currently through Paris) and stay in Munich for 1 day? Will he need any invitation letter from me to show at Munich Airport?

He can stay in munich provided

He has a schengen visa issued by a EU country and is valid during the time of stay in Germany.
He wont need any invitation letter

Thanks. My friend did not face any questioning from Munich Immigration Police.

ah, i can post this bit of information regarding the Schengen agreement since it came up in conversation yesterday.

if you have a visa for the Schengen countries & if you are also from one of those countries that needs a visa wherever you go (for example China or Taiwan or maybe even India) and you want to fly to the U.S. on holiday and have a transfer in the UK, you will need a special transit visa for the UK. if not, it is possible that when you go to board your flight in Germany, they will not let you because you do not have the proper documentation.

this happened to an albanian roommate of mine who had permanent residency for germany, but not yet a german passport. he was on his way to a student exchange in the U.S. and had a layover in Heathrow & he was not allowed to board his flight in Stuttgart. also, this is a situation facing a friend from Taiwan who is studying here. she was not aware of this either.