If the person is attending because he/she agrees to their stand that there is no valid pope since Pope Pius XII died in 1958, that would be considered sinful; but if the person is attending for educational or social reasons, that wouldn’t necessarily be wrong.
It all depends on why the person would want to attend a sedevacantist mass.
Raphael Benedict is a Catholic who wants nothing but to spread the catholic faith to reach the ends of the world. Make this possible by always sharing any article or prayers posted on your social media platforms. Remain blessed