He needs the treble clef and you need the bass clef.
Lead sheet will give him the treble clef and you have the chord name to build your bass line around.
Google - Lead sheet, "name of the song"
Christmas carols should be easy and have what you want.
http://www.8notes.com/scores/1461.asp Low note in the bass clef is what you want for your bass line. Here is another
http://www.guitarchordsmagic.com/gre...heet-music.pdf Notice the guitar tab - bottom four strings are the same as your bass ...... interesting fact, however I bet those tabs are melody and have nothing to do with the bass line you need. I did not check.
Your music store will have "piano sheet music" find some you both would like. Second hand book stores normally have sheet music at bargain prices.
If not Christmas Carols, early Rock and Country should be easy. See what Google can find. Your church hymnal should have some Christmas Carols with both treble and bass clef. And some will probably even show the chord names.
You may have to transpose the key to whatever his trumpet is/likes, i.e. Bb, etc. But, he should know about this and can guide you. Talk to him about that. I go Jell-O on how this works for the trumpet, but, it is something you will need to address.
Good luck.