Much as I love the organ, from the OP’s post, his friend is a gifted pianist and probably has volunteered in love to play for the wedding…
And a pianist is not an organist, by any stretch. A piano player can often play one of the ‘electric’ type organs passably, but seldom ‘well’ unless he/she has actually studied organ too. Especially the type of ‘church’ organ that most likely is available. . .with keyboards, stops, and foot pedals. . .and sometimes with bellows that need to be pumped by another person as the organist plays! And he/she certainly cannot do the pedal work that is one of the beauties of most works for the organ. Even the ‘touch’ required is different; a pianist relies on the sostinado or sustain pedal to ‘blend’ notes but an organist works more with the fingers and ‘timing’ and the stops to create blends, and the characteristics are quite different.
So the OP’s friend, a pianist, would likely not be able to play the organ well (especially if he is a gifted pianist, believe me, he’d be the first to recognize how far ‘short’ his playing would be on the organ).
In this situation I can certainly understand that in order to allow the pianist to give his ‘gift’ to the bride and groom, a piano would be needed.
And before some say, “Can’t he play for the reception?”. . .reception playing is yet another ‘field’. A gifted pianist (probably classical, if he is considering playing for a wedding ceremony) is again a whole 'nother animal from a pianist specializing in receptions. Not that they aren’t gifted themselves (many are) but the reception is usually the place for ‘show tunes’, pop, ballads, even ‘funk’ or country depending on the bride and groom’s taste–at most ‘light’ classical and only limited at that. Probably the pianist in mind would not work well in that atmosphere at all.