fortunately this was only a pre-game discussion, but we did not come to a clear resolution in my mind.

In NFHS, if there is defensive pass interference with a completed pass, does the offense have the choice of taking the result of the play or 15 yards from the previous spot?This is the way I believe it should be.

Or is it that the penalty is enforced from the end of the run after the catch. I find nothing that supports this, but was told it was the latter and they were sure.

Thanks for the help

The penalty can be declined; then the result of the play would stand. If accepted, it's a foul during a loose ball play, so it'll be enforced from the previous spot.

Check 7-5-10 and Table 7-5.

You are correct. There might be some confusion with intentional pass interference which is an additional 15-yards, try calling that, or roughing the passer with a completed pass which you tack on an additional 15 yards.

Canadian Ruling
 

Accept the penalty or decline the penalty & accept the result of the play.

PI by the nature of the act occurs during a loose ball play. The basic spot for a loose-ball play is the previous spot. Fouls by the defense are enforced from the basic spot (all but one principle). There are exceptions to this but PI is not among those listed.