I'm sorry to hear your relative is so poorly.
Plants that are rooted in the substrate need root tablets. Some are better than others. For example, the API ones are notorious for disintegrating and making a mess of the water. If you can find them, Seachem root tabs seem to be one of the best.
If you have any plants like java fern or anubias, they can't be planted in the substrate so root tabs are no use. these need a liquid fertiliser that is added to the water. Seachem Flourish, with nothing else in the name, is one of the best as it contains virtually all the trace elements.
Aquarium plants like garden plants need both micro elements (trace minerals) and macro elements, the NPK that garden fertilisers talk about. Once there are fish in the tank the nitrogen of NPK comes from the ammonia made by made by the fish, and phosphorus and potassium are in fish food. As the moment you have no fish, so adding trace elements won't help. Until you start cycling properly it may help the plants if you add a bit of fish food - this will need to be removed when it goes mouldy, and it will probably increase the ammonia reading as well.