Charlie is a very lucky bird to have found your family and I am glad that she is receiving so much care and love in your home.
Psittacine Beak and feather disease (PBFD) is due to infection with a virus and affects the immune system, feathers and beak of affected birds. Normal bird beaks do not require trimming due to a balance between growth and wear however beak and feather affected birds will often be unable to wear down their beaks properly leading to an inability to pick up seeds and grain and associated malnutrition. The beak is a highly vascular and sensitive structure with PBFD affected birds being at increased risk of infection.
For this reason trimming should be reserved for a veterinarian. I would recommend taking Charlie to your local bird vet and asking them to demonstrate their technique for beak trimming and decide after this if you all feel comfortable with continued at home maintenance of Charlie’s beak. Best of luck!