A 45-year-old patient taking Lithium Carbonate MR 400mg twice daily for bipolar affective disorder has a stable serum lithium level of 0.7 mmol/L (target range 0.6-1.0 mmol/L). They develop gastroenteritis with significant diarrhoea and vomiting for 48 hours and concurrently start taking Ibuprofen 400mg three times daily for associated muscular aches. What is the most significant potential risk in this scenario?