How should a Muslim who wishes to apply the Sunnah act if he visits people who do not apply the Sunnah? Should he sit on the floor or follow the custom of the household?

The Prophet, peace and blessings be upon him, abandoned eating from a dining table (khawan) and thin bread out of humility and asceticism. Eating from a dining table is permissible, and one who does so should not be criticized. If one is invited to a meal served on a table, he should eat with them. However, if he explains to them the practice of the Prophet, peace and blessings be upon him, and reminds them of asceticism, that would be good.