Psoraisis inside mouth

Hi hope someone can offer some advice. My mum was recently diagnosed with Guttate psoraisis presumably after a virus. She's 60 and never had any skin problems before. She has lesions on her arms, legs, back, shoulders, soles of her feet, toes and palms of her hands. But the worst is lesions inside her mouth. She's struggling to eat or drink anything because it's so painful. The GP gave her a mouthwash but after two months, still no improvement. Does anyone have any tips on treatment or something that can ease the pain? Thanks