It can be enjoyed either way but warm I think will bring out the flavor a little more. Depending on the type of fruit cake I think some are wrapped up and steamed to heat through but I'm sure microwaving a slice is perfectly fine. I'd suggest just heating it a slice at a time if microwaving and maybe on paper towel to avoid sogging

Since you mention it had dollops of caramel though, heating might cause issues there since those would melt?

Personally I'd try a bit both ways and also dipped in the coffee and just see if there's a way you prefer.