I’d email the teacher directly, asking for support materials, revision work etc. Put the ball in their court asking for things that you can do to support her. Also ask what specification they are doing and the topics. They should be able to provide this easily. If it is Edexcel, I may be able to grab some bits for you from my department and email them over to you.
I’d worry that if the teacher is not there that often, yet she is still on target, then the teacher is putting in grades on the system that matches the targets, so it looks as though their absence is not having an impact. See if she can bring any marked assessments home, then you can get a good idea of the work she is producing.
If the work is not forthcoming, or the assessments do not match up with current grade, email the HoD informing them of your concerns.