Is the questioner sinful for causing his friend to abandon prayer after he had only been praying during Ramadan?

One who abandons prayer while acknowledging its obligation is in grave danger, for prayer is the pillar of Islam, and severe warnings have been reported concerning its abandonment. The Prophet, peace and blessings be upon him, said: "The covenant between us and them is prayer; whoever abandons it has disbelieved," and "Between a man and polytheism or disbelief is the abandonment of prayer." Praying only in Ramadan is not sufficient; rather, all obligatory prayers must be observed at their appointed times. Advising one who abandons prayer and reminding them of these texts is a duty. What you said to your friend, that he is "mistaken," is not blameworthy, because your intention was that he is wrong in limiting his prayer to Ramadan.