OK, so you transpose a tune from F to G because it is more convenient to play in G. That's a different matter than playing a recorder as a transposing instrument. I quite often do that with Playford tunes so they can be played on common folk instruments such as whistles and diatonic squeezeboxes. Not a problem.

You can then take the tune in G, transpose it down a fifth (seven semitones) (Tools>Transpose Staff in Noteworthy) so that you have it written out in C so your friend can play his F recorder as if it were a C instrument.

Fine. It will work and as a quick fix because, for example, you need him to play with you this coming Sunday on a instrument he is not familiar with but is similar to one he already knows it is useful.

However in the long term, should he meet music specifically written for Bass recorder in bass clef, he will have problems, so it is in his longer term interests to learn to play from music written for the bass recorder in bass clef. As I said earlier it does not take that long. I think I was the one who suggested two weeks and that was not for a 14 year old. I was in my late 40s when I took up bass recorder that's a long way from 14. If you know the fingerings for any recorder, then it's a matter of making different associations rather than learning new fingerings. I admit the two weeks was to get over the initial "hump" and you do make mistakes after that. I still do some 20 years later, especially when changing instruments for different pieces.

Learning all the different ways of reading music for the recorder gives you a lot of flexibility. I am currently playing the bottom line of an arrangement of a slow air which was originally written for four fiddles (the arrangement, that is). It means treating my bass as if it were a treble recorder reading up an octave. No problem if you learnt the F recorder relationships on written music. It's like a lot of things: Learn the rules properly then you will know when you can break them.