I think you should tell the teacher. It's just wrong to claim somebody else's faults as your own and feel guilty about it. Some people just need to get it the hard way before they understand something should be done different. Of course it's sad if that person does not pass his maths tests, but you should not be the one responsible for it. With teamwork you should be able to rely on your team-mates and if they continuously fail to do their job, they should be kicked out of the group.