Okay, before the torches and pitchforks are broken out...
A relationship is a two way street. Both sides need to communicate and actually say something is wrong. After all, no one has true telepathy (...yet). So people will default to the old "I know you'll do great".
Is that helping Claire right now? Probably not, but Marten hasn't done anything wrong here, yet. He's trying to be a good boyfriend, giving his girlfriend the encouragement he thinks she needs. I mean, stressing about exams is a common thing to stress about and most of the time, people just need that encouraging word.
The problem would be if Claire kept trying to talk to Marten and he just keeps saying encouraging things, without taking the time to actually listen to Claire's problems and fears, then talking through them with Claire.
Remember, Marten is a decent person, but he can't read people's minds and instantly solve their problems.