*law enforcement background*
Based on what I can see as far as what's been made public, no this isn't a manslaughter case.
There's two forms of manslaughter (voluntary and involuntary) Voluntary manslaughter would be if two guys got into an argument and it escalated to somebody shooting the other party. Since the shooting wasn't planned and only happened in the spur of the moment it lacks the mens rea (guilty mind) component of a murder which by definition is premeditated.
Now that could change if say the victim was black and you could prove the shooter chose to escalate the situation because he was a white supremacist or something. So can a murder be spur of the moment? Sure but you need to find another way to exemplify the mens rea component like I just exemplified.
Involuntary manslaughter on the other hand is actually easier to successfully convict and that's because there isn't that grey area when those cases go trial where a jury can debate if a case even warrants a murder charge or if manslaughter really should be murder. IE Casey Anthony. Had that case just been tried as involuntary manslaughter it was a slam dunk. Mom likes to party, mom loses track of child, child ends up dead. It's manslaughter because she was responsible for the child, and her lack of responsibility resulted in her child's death without that intention. Now obviously in that case there was more to her just losing track of the child but what actually happened wasn't something that could be supported by evidence hence why that case should have just gone to trial on what was actually tangible. DWI cases are another good example. Guy drives drunk, and since he knows by default drunk driving can kill another person although it's accidental his action caused the other party's death and his CLEAR lack of regard triggers that mens rea element there.
In this case though, you had a party of upwards of a thousand people, where everybody was partying, and one fucked up dude jumped off a roof and landed on somebody else. This isn't like the DWI where whoever jumped had an existing responsibility of knowledge that bound him to "knowing better" in regards to knowing whether anyone was in the pool, nor was it the owner's responsibility to assume anybody would jump off whatever this guy jumped off. So regardless of who you go after the mens rea of involuntary manslaughter doesn't exist.
Now if the pool was hosting swim trials and a guy scaled the balcony and jumped in there and killed somebody it would be different, because he understood he was risking the safety of others. But as the article mentioned the pool was littered with debris at the time. It's not what you it's what you can prove.....beyond a reasonable doubt.
Now obviously you can sue civilly for anything and maybe Keenan's family ultimately went that route but this wasn't a criminal case in way shape or form.
Sad tragedy, but that's what this was.