I would write to the instructer today and let him/her know what's going on and that you will do your best to cover the material. Then use any extra time you have to fill in their slides. if they don't get anything to you the teacher knows you did all the work. If they do you can let them explain their slides thereby showing the teacher they did come through. Communication is key here. Your failure to communicate in advance to the teacher will most likely result in you being held accountable.

Part of group work is to build communication skills helping us navigate issues such as this. Work it out to the best of your ability but keep everybody informed.

Love and hugs,
Tara