You guys do realize the plant is in a POT and water behaves much differently in containers than it does in mineral soils - right? Since Leena's mom cares for the plants, checks the soil with her finger, and
waters when the top couple of inches of the soil is dry, watering it every day or every other day depending on the temperature, AND, the medium looks rather water retentive, it is very likely she is watering when there is still a significant volume of water remaining in the grow medium, which doesn't sound anything like under-watering. Leena's pot is about 12" deep, so the top 2" of grow medium can feel completely dry while the bottom 6-8" of the soil column could still be 100% saturated, especially if the medium is as water-retentive as it looks.
Additionally, we don't know if the home might have an ionic exchange water softener, or if a high TDS/EC level is in play, or low fertility which would also account for spoiled foliage, even use of a high phosphorous bloom booster fertilizer could account for the conspicuous symptoms.
@LeenaY If your mom's watering intervals are short enough that she is watering while she can still detect some moisture in the medium with a finger, there is almost no chance she is under-watering, and a very significant chance she is over-watering. Over-watering causes the same drought response as under-watering because the lack of oxygen in the rootzone (which accompanies over-watering) limits the plant's ability to take up and efficiently move water to the plant's distal parts, usually leaf tips and margins.
My suggestion would be to have your mom start using a wooden "tell" to monitor moisture levels deep in the pot. You might find she's watering at intervals too short, or even intervals too long, but at least you won't be jumping to a conclusion and ignoring other reasonable potential causes. What can you tell us about fertilizing? Does your home have a water softening system (ionic exchange) that uses salt as the exchange medium.
Using a "tell'
One of the most reliable methods of checking a planting's need for water is using a 'tell' (more reliable than a 'moisture meter'. You can use a bamboo skewer in a pinch, but a wooden dowel rod of about 5/16" (75-85mm) works better. They usually come 48" (120cm) long and can usually be cut in half or in several pieces, depending on how deep your pots are. Sharpen both ends of each tell in a pencil sharpener and slightly blunt the tip so it's about the diameter of the head on a straight pin. Push the wooden tell deep into the soil. Don't worry, it won't harm the root system. If the plant is quite root-bound, you might need to try several places until you find one where you can push it all the way to the pot's bottom. Leave it a few seconds, then withdraw it and inspect the tip for moisture. For most plantings, withhold water until the tell's tip comes out nearly dry. If you see signs of wilting, adjust the interval between waterings so drought stress isn't a recurring issue.
FWIW, I have grown plumeria, but not by choice. I was given trees by Laura Jones and the late Bud Guillot, both internationally known for their named varieties and personal contributions to the plumeria community. I also provided my dear friend Laura with several years of guidance re grow media, nutrition, and other cultural issues. I've also repotted (full repots) dozens of plumeria for others, family/ friends/ business associates.
I've been studying plant sciences and soil science for almost 50 years with a focus on sharpening my ability to maintain extremely healthy plants of all kinds in pots. Two things I learned early on are not to jump to conclusions, and one needn't be a bus driver to know what makes the wheels go 'round and 'round.
Al
