Making sure I'm reading correctly: Two weeks per month a 50% water change; one week a 100%; one week no water change. So, three water changes per month.
100% water changes are extremely stressful for Betta and if the tank has a filter are unnecessary. Anytime a Betta is removed from its tank is stressful. A weekly 25% water change and vacuum is better; leaving the Betta in the tank.
His symptoms could be any number of things. Until we can figure it out I would suggest adding either Indian Almond Leaves or plain Rooibos Tea for their antibacterial and antifungal properties. Tank should be at the least the color below. If floating a leaf or teabag doesn't achieve this tint you can brew a strong tea, condition and add to tank. Leftovers can be stored in the refrigerator.
As noted in the form, we need a photo from above. This is so we can determine if a Betta is receiving enough nutrition.
A side note: One of the reasons I personally do not like flakes is you can't tell how much a fish is eating. With pellets you get an exact measurement. If your cousin will switch to Fluval pellets it would help her determine if Bibble is getting enough to eat or if she needs to make adjustments.