Jamaat Salaah and Imaamat

Not joining the jamaat Salaah

Q: This weekend I was out musafir for a few days with a few friends. We came to a masjid to perform salaah. Two friends we finished making wuduh and the third was still making wuduh so the two of us decided to start our farad thinking the 3rd brother will join us. Our first rakat was almost finished when the third brother finished his wudu. The brother didn't join us but performed his salah on his own. When asked why he didn't join us he said because we didn't wait for him. Is this brother sinful for doing that?