Perhaps I'm missing the wider context Terence but I can't see how this is anything other than 75% your colleague's fault for assuming an instruction was received and digested. It's naive to assume using an IM on today's collaboration tools means guaranteed delivery AND viewing AND acceptance of the instruction. Plus it doesn't take long to realise Google chat is a bit shit. You've already suggested there are multiple IM options used in your organisation, plus there's email, meetings, texts. The cognitive load is wide, and it's not like IMs are a Kanban board where you pull from tasks to be done. Most middle or senior managers asking a colleague to do a task would ask a follow up if it was important and their colleague hadn't replied to the message. If you don't get at least "ok" in response then you should assume it hasn't been received.