No, it would be cowriting credit because she wrote the piano. Jenkins doesn't play piano.
There is a difference between writing the music and lyrics-I have seen plenty of cds that denoted that someone write the lyrics while someone else did the music. If it does NOT explain that, then its implied that both aspects are cowritten. (For example, in one Hanson cd it says lyrics done by ITZ Hanson/cowriter, all music done by Hanson)
Does it say that in the Harmonium sleeve? It would say, lyrics by Stephen Jenkins, music by V Carlton.