Does your Dhamma friend happen to specify which cetasika is prevalent during 'imagination'?
I think the more relevant question is : What is the nature of the object that the citta takes?
See Bhikkhu Bodhi's notes to the Ahidhamma Sangaha (Comprehensive manual of abhidhamma).
III.17 (page 136)
For all types of eye-door consciousness (citta), visible form alone is the object and that pertains only to the present, ... [other sense doors].
But the object of mind-door consciousness is of six kinds, and the object may be past, present, future, or independent of time, according to circumstances.
Further, in the case of the door-freed-conciousness --- that is rebirth-linking, life-continuum, and death (conciousness) --- the object is sixfold, and according to the situation (that object) has usually been apprehended in (one of) the six sense doors in the immediately preceding existence, as either a present or past object or as a concept. It is known as kamma, or as sign of kamma, or as sign of destiny.
Some of the notes:
And that pertains only to the present: [The point here is that the objects of the physical senses are what is actually here now.]
The objects of mind-door consciousness: The cittas that arise in a mind-door process can cognize any of the five physical sense objects as well as all types of mental objects inaccessible to the cittas of the sense-door process. Mind-door cittas can also cognize an object belonging to any of the three periods of time --- past, present, or future --- or one that is independent of time (kalavimutta). This last expression applies to Nibbana and concepts. Nibanna is timeless because its intrinsic nature (sabhava) is wihout arising, change and passing away; concepts are timeless because they are void of intrinsic nature.
So cittas can have paramattha (ultimate) dhammas as objects (sense objects, nibbana), or they can have conceptual objects (pannatti). Conceptual objects include all kinds of thinking, etc, and also jhana objects.