If the fish isn't eating, it could possibly be constipation. Is the betta swollen around the abdominal region at all? How often do they change the water? What size tank is it?
It might be best if your friend did a 100% water change, making sure they match the temperatures of the old water and new water exactly and not feed the fish for 4-5 days. After starving (aka fasting) the fish, offer a bite size piece of the inside of a defrosted pea for 2 days. If that doesn't help, an antibiotic such as tri sulfa would be the way to go
Looking forward to your answers and reply!