"I'll ask him when he drops by for a cup of morning coffee." The reason he drops by? For coffee. "On" sounds rather strange, and only makes sense if the morning coffee had a powerful drug-like effect on him (and he drank it before he dropped by). As if he were a caffeine addict and was "powered" by the morning coffee he had drunk.

Hi Jinx: You would never use the English word "on" in this context. Some people might say something like; "I am ON my first cup of coffee". That however, is an awkward usage, because we are not ON coffee, nor IN coffee. Such confused usages are clarified when we write or say things on the subject, like this; "I am drinking my first cup of coffee." . So, for the sake of clarity, just eliminate the use of ON in this context, and you will have no difficulties. .

its for my friend