Tin has a melting point of 231.93°C. Brief exposure to this might not char a horn, depending on the nature of the horn. If the horn is relatively thermally conductive then it might well not char, or not much. Especially considering that the tin will tend to stop any oxygen being available and so slowing the char process.
Some types of solder are alloys of tin and various other materials. Some types of solder have melting points as low as 90°C. These are usually less shiny than pure tin. It's possible that, with some tweaking, a brightly shiny form could be made that also had a relatively low melting point. There would need to be some development effort to get something that looked good but didn't simply abrade off in a couple days. Maybe the keepers of these critters care about not poisoning them, so they make an effort to select less toxic materials.
The thing is, horn isn't typically dead and un-growing. It's usually much more like fingernails. Think of hair growing, or a horse's hooves. They typically continue growing because they are expected to be abraded, broken, etc. Some species will shed them on a yearly cycle, and regrow them, possibly at rutting season or some such. Some species will be constantly growing their horns, with new material appearing at the base similar to finger nails or hooves.
So it means that the dip will probably need to be replaced on a regular basis. Depending on exactly how the horns grow it might only be once per year. Or it might require a process similar to "touching up the roots" for people that color their hair. There will also need to be touch-ups if the individual gets into some rough situation and abrades off the coating.
Wheezing off another answer: Probably the horn needs some prep before coating. Certainly it will need to be cleaned. Maybe it needs to be sand-papered, or abraded. Or possibly it needs to be chemically treated first to give the surface a nature that will accept the coating. This would all be part of the ritual.
Further edit: The temperature of the molten metal is certainly going to be enough to harm the fleshy parts of the Tiefling. Even if one of the very low temperature alloys was used. Possibly you are going to want some kind of wrap or ring or something around the base of the horn. This will cover the last small portion that would not actually get dipped. It could be of the same metal or cloth or leather or some other material. You could play a lot of symbolic games with that, it being an obvious symbol of being bound. The ring color could indicate status or rank or some such.