Hiro was a a jerk not contacting her and neither telling her se was back in town, but let's not assign all the blame to her. Both parties can try to call, but we aren't neither shown or told that Chiharu tryed to call Hiro herself. We don't even know how much time passed from the no call period start
Hiro probably didn't know that Chiharu was insomniac and was taking sleeping drugs, that were probably the cause of death by drowning.
If there is one to blame for her death I feel is the doctor providing the drugs, that could not have explained the risks of taking baths after taking the dose, or the mother that could forgot to convey the info if the doctor said it to her. Many medicine doesn't actually say in their readme what are risky behaviours. Some do have an average effectiveness time (for how long the medicine have effect after taking).
A bit of commentary on real life. I honestly was curious how you can drown in the bathub and if it was actually common:
Falling asleep (and not fainting) in a bathub generally doesn't result in drowning even if your head went submerged. The reflexes would wake us up, and would prevent us from breathing water until the reserve of the lungs is depleted. Some drugs increase drowsiness and reduce these relfexes (either as a side effect (that can not happen always) or as a primary effect, as most sleeping drugs) causing either breathing water or removing the lung spasms. These effects aren't only limite dto drowning, I remember when I was having trouble sleeping, and was taking a pill. I recall how many times I woke up with my bladder basically ready to explode when I generally woke up way before it can to that point. Once I didn't woke up at all.
Alchol (intoxication or not) can also increase these risks a lot. Avoid taking baths in these conditions , unless you have someone that can act a supervisor, prefer waiting for the effect time of the drug to pass (some have it on the manual, most common use medicines have the effect time generally available on reputable sites) or better a bit more (being the average time, the can be people were the effec ttime is increased), and still avoid if you are still drowsy.
Risks are modified on how the bath is made. From what I understand Japan common bathubs are deeper then overall western bathubs. This is suspected (togheter with a lower median height of the populace) to the be one of the main factor of the astounding percentage of bathubs related death in Japan (60% of all yearly deaths ) related to other countries (the next position is occupied by USA and Canada with an 11%). Possibly becouse there need more events to have the head submerged when the bathub is more shallow. Anti slippery base also can help reducing a lot the risks.
These measeures can also improve in some cases of fainting (due to change in blood pressure, hypertermia due to too hot water(for too much time), too much time itself, or cardiac arrythmias (self terminating or even atrial non self terminating ones, in this cases the fainting is caused by sudden change in beats and blood pressure that the body start to compensate soon in most cases )). Not much for strokes or hearth attacks (just hope you have time to call someone and that there is someone at home and in hearing range, fainting or death are not immediate in most cases) or sudden cardiac death. You don't have the time to drown here. The proability of these events can be increased by prolonged bath times (health professionals recommend 15 max 30 minutes, if healthy ) and high temperature (most guidelines recommend max 39C degrees, some professinonals says maximum 40C as peak if it's not continuos (so if temperature decrease with time, no new addition of newly hot water))
Hot baths can increase the possibility of orthostatic fainting or dizziness (orthostatic means changes when standing up) and their severity if you are already esperiencing them (form example standing from a bad or a chair, often when done too fast). Less hot water and less bath time can reduce the risks, also avoid standing too fast, make slow movements if you can. The risk here is being injured (for example on the head) and that you can end with your face underwater (heads or spinal injuries can increase the drowning risks). Same risks for seizures where the loss of consciousness already have a risk of drowning but the jerking movements have an high posisbility to make injuries. Also muscolar movements (unless the seizures are atonic) can help in certain case to push the face underwater, increasing possibility of drowning even with antislippery bases and shallower baths.
Also most of the deaths in Japan (about 80%) are from people of age 60+. Causes weren't scored, but I suspect that this is becouse this is the it start the age range were people have more probability of having new onset cardiac related or blood pressure related illnesses or start using drugs for other new onset ilnesses.