Tenant complaining about limited hot water

[Maxwell Servais]

I have a tenant in my 4-unit property saying that hot water runs out after about 10 minutes. I have turned up the water heater to the highest temperature available, but it does not seem to have fixed the issue. None of the other tenants have mentioned anything about hot water running out, but this tenant says she used to have enough hot water to bathe herself and her dog, and now it runs out after about 10 minutes. 

Any advice on some additional improvements that could be done to potentially fix this problem without having to buy a new water heater? Water heater is about 5 years old. Thanks in advance!