No, you miss the point, so let's try and break it down, because I'm afraid that you're asking the wrong sort of question.
A towel rail with an element will never have an element in it that would allow it to reach an unsafe surface temperature.
There has to be a common reference point, and that's measuring the rad (either water heated, or electric heated) in BTU's. A plumber would measure the size of the room, and then take into account whether it's got solid walls, insulated or however it's constructed, and they'd then calculate the number of BTU's required to heat the room. But towel rails were never designed to heat a room, (unless it's quite small) they were designed to warm up the towels, and having a heating element in them, they could do it all year round, not just when the heating was on.
A central heating rad will be running at about 75C, and a towel rail with an element in it will end up with about the same surface temperature, but a larger towel rail, with a larger heating element will be able to heat a bigger room. You're not going to be in a situation where you get a towel rail that was designed to take a 300w element, then having a 600w element in it, as this would definitely overheat the rail. A 600w towel rail, will be a bigger size than a 300w rail, so there shouldn't be a problem.
You're friend has to work out what size rail/element/BTU that he wants, either to heat the room, or just warm the towels.
Hope this way of explaining it helps
