Tenant asks to replace working baseboard heaters though the work

[Al Max]

My tenant asked me to replace baseboard heaters that are old, but work fine. She claims that the temperature never goes over 65F and they are not efficient. I came and checked all the heaters and they got evenly hot and work fine. I used to live in the apartment myself and it takes between an hour to a couple of hours for the temperature to get warm once you turn them on, depending on the size of a room. Additionally we don't have too much cold weather in Northern CA. The tenant was a bad news from the beginning and now she's loosing it. She threatens to sue me and so on. I don't want to replace working heaters, but can it become a bigger problem? Is there any remedy for this situation?