TaranTheWanderer
Legend
An aboleth and an illithid walk into a bar....
In all seriousness, here is my guideline for a "reasonable" suggestion:
A reasonable suggestion is one you might possibly convince the target to do without using magic, even if it's extremely unlikely. An unreasonable suggestion is one that there's no way the target would ever do without magic.
Were the PCs talking to the aboleth at a bar?
A demand is a forceful suggestion.
Would it help if he said 'I suggest you foojooble off' instead? I mean come on.
'Foojooble off' is as legit and reasonable as 'Go away' is. That's clearly what it meant and you can only argue otherwise from an obtuse position.
The other issue is not only can the aboleth not hear the suggestion, but as noted in post #7, the PC cannot telepathically communicate with the aboleth to give the suggestion. Also, suggestion requires you can see the aboleth and it is not apparent in the OP that the PC's can even see the aboleth.Well it is verbal in the ordinary sense. The question is whether you are able to hear and understand a telepathic comminication. Id say you are able to do so because 5e is written in natural language and those words are exactly what I would use to talk about telepathic communication in natural language.
The other issue is not only can the aboleth not hear the suggestion, but as noted in post #7, the PC cannot telepathically communicate with the aboleth to give the suggestion. Also, suggestion requires you can see the aboleth and it is not apparent in the OP that the PC's can even see the aboleth.
A reasonable suggestion is one you might possibly convince the target to do without using magic, even if it's extremely unlikely. An unreasonable suggestion is one that there's no way the target would ever do without magic.
What it is, is the Jedi Mind Trick.
You shouldn't be able to pull off "Abandon this fight over a major magical artifact, turn around, and walk into the trackless wilderness for an entire day." (Yes, I had a DM do that to me. I'm still sour.)