You don't indicate a particular purpose for getting your friend "playable" on a whistle. I haven't seen any such thing, but that doesn't mean they don't exist.

If it's just a matter of the two of you playing together so that you can learn his tunes, perhaps he could use a "practice chanter." Elderly instruments (as an example only) has these
HERE for about $25(US).

If you are wanting to get him into a group, you would need to consider their attitude(s) about such an instrument usage, and whether "pipe tunes" fit in with what they want to play; but that's something you would need to settle with the group.

If you really need a "whistle," I'll defer to those who know more about pipe fingerings and whistles than I do.

John