If you have two devices running iReal Pro, make sure the same songs are on both and are set to the same key. Set the trumpet device to global transpose to Bb. (Information in More>Tutorials.)
If you only have one device, you might be able to easily print out one instrument's copies of the songs in its key, then use the device for the other instrument; or email some PDFs to another device that does not run iReal Pro.
When you are not available to practice, he can set iReal Pro to global transpose Bb, and use the player to practice with.
Remember setting the key in song view (bottom right) is the concert key. The global transpose (Bb for trumpet, or C for your piano) will change the chords for the that instrument if necessary.
If the player is used, it will play in concert (piano) key (ignoring any global transpose) but showing the transposing instrument key.
If you want a weird way of reading off the same device for both of you, turn global transpose to Bb, choose the key for the song, change the font to piano. The trumpet would read off the (smaller) chords above the piano diagram font, but you would read the diagrams (it shows where your fingers go, ignore the chord symbols). It could be fun for you if you are getting bored.
Or you can use a split screen (iOS 9 later devices) where one side shows iReal Pro and the other a PDF of the chart for the other instrument.