In my opinion, you need to be specific about what the silly mistakes are. Are they always in the same types of question (fractions, symmetry, algebra etc)? If so, obviously she would benefit from extra practice in that area/those areas.
If they're basic addition/subtraction/multiplication/division errors when working at speed (and when you think she should know better), it's worth going right back to basics and working in some time for practice of number bonds up to 20 and times tables. These need to be really secure.
If the mistakes are caused by rushing, or panicking, or both, then allow her more time for her practice papers. Don't be fooled into thinking that the more practice papers you do the better. She needs to go over the areas she's unsure of and consolidate the knowledge she has, not just do paper after paper without learning from mistakes.
Finally, teach her (over and over again) that checking means doing the question again.
I hope this helps and best of luck to you both! RR