The usual culprits are the strep or staph germs. They can enter the skin through a tiny scratch or puncture. Diabetes makes a person susceptible to cellulitis infection. Once the germs get a toehold in the skin, they spread rapidly. The skin turns beefy red and is quite painful.
Antibiotics can usually put an end to the infection.
In serious cases where there is widespread involvement, the doctor infuses antibiotics by vein. Having cellulitis for two years is unusual. If you mean that the infection has recurred often in the past two years, that's understandable. Her diabetes would contribute to repeat infections. Thank you for the photograph of your friend's legs.