I think the misunderstanding comes from the way the question was worded: "how do I go about transposing the soprano part to alto". The way I understand it there's no transposition involved. The music is supposed to sound more or less as written and Jeff is playing the alto as it's supposed to be played, i. e. alto fingering. Personally, I've stopped worrying about octaves a long time ago (soprano recorder and bass recorder are notated an octave lower than they sound, guitar on the other hand is written an octave higher than it sounds - who cares...)
For me, there is only a problem if the alto's range clashes with the notes required in the music (low c, d and e - and corresponding half tones). In which case you need to find a solution, such as:
- playing one note an octave higher
- playing a few notes or the whole phrase an octave higher, so the one unobtainable note doesn't "jump out" so far
- play another note than the one that's written (ideally one that the guitar is playing, that way you can be sure it fits into the harmony - or ask the guitar player for suggestions, if he knows more about harmonics than you do. Or experiment.)
- if the whole, or most of the piece is in the soprano's lower 1 1/2 octaves, you can play it as if it was written one octave higher - though that might lead to needing to play the highest notes lower than written
As for other sheet music for alto recorder and guitar, you could have a look at these:
https://imslp.org/wiki/Category:For_recorder,_guitar - they can be downloaded for free (you'll just have to wait a bit if you are not a paying member. And of course the archive could do with your financial contribution...)
I have Dmitri Schostakowitsch (I know he's spelled differently in English but that's what's written on the sheet music) Walzer Nr. 2 aus Suite für Variété-Orchester, Universal Edition EE 37 146. ISBN 978-3-7024-7480-5. Universaledition claim to have a New York office, so you might find it.
www.universaledition.com (That's the waltz that made André Rieu famous when he played it in Munich's football stadium during half-time break. I still haven't played it, but it looks easy enough - maybe too easy, i. e. boring, for guitar).
I also have a book of easy Christmas songs for guitar, and whereas I still can't play them on guitar, I've found that it's actually easy enough to find the melody line and play that on the recorder. Of course, with Christmas songs that's especially easy since we often know them, but you might ask your friend whether he has some nice easy pieces with an easy to find melody and just try them.
Last but not least, if your friend can play from chord symbols (i. e. the letters over the notes - so far I've assumed he is a classically trained guitar player) the whole world of song books, fake books etc. is open to you. The only problem is that you'll never know which ones are easily playable for alto recorder without looking at them (I'm a big fan of the Edward Jones Huws "Fiddler's" books published by Boosey & Hawkes, but I play them on tenor recorder - and some collections are more suitable than others). Do you have any music shops within reach?