I like the idea of a diary entry, but instead of the introductory sentences explaining the diary entry you could mention that Pierre was writing in a diary earlier in the story. Jemima could tease him about it and Pierre could get embarassed. Not only would it make the transition to this new point of view less awkward, but it would also reveal more about the personalities of the characters.

Oh, so you mean I mention it in the earlier chapters? Okay, that sounds good.