I believe both SF and CEP support multitracking. I've never done multitracking before so maybe others will chime in with more detailed info, but I'd do something like:
[1] get the karoake tune onto PC as its own two-track
[2] play back the karoake on his headphones while he sings and you record
[3] play back the karoake on his headphones while he plays trumpet and you record
[4] master in SF or CEP - align tracks, adjust relative levels, etc.
[5] mix down to two-track
To enter the multitrack view in CEP, on the main menu: View > Multitrack View. CEP has pretty good documentation - try starting there and learning as you go.