The back vents on the retina models are for exhaust. The side vents are for intake.
I’m quite sceptical about the Apple Genius’ statement about there being coffee all over the inside of the product. Unless, of course, we’re talking about a significant amount of coffee, and it splashed in the direction of those vents. 125ml? 50ml? Was it full cup, or an almost empty one? More? Less?
Take the bottom off and inspect it yourself. Keep in mind that you’re only going to see one side of the innards, but if the Genius was truthful, “all over the inside” would probably mean that you’ll see some corrosion or dried coffee even on the bottom half of the inside.
Personally, I suspect the Genius, at best, only took the bottom off and spent a few seconds giving the bottom side a cursory glance. At worst, they didn’t even take the bottom off, and just gave you a canned response. To accurately state that there was coffee “all over the inside” (meaning on both sides of the internal components,) they’d have to take the logic board out, and that takes a while to do.
Can’t really speak much to family issue though, aside from what i would do if it were my own. All I can say is that if it were me and my family, I’d know that my sibling would probably feel absolutely horrible, but they definitely wouldn’t be able to afford such an expense (assuming the Genius wasn’t giving a standard line response, which I highly suspect they were.) Knowing that, and the risk/impact of a liquid spill happening under their watch, I’d probably never let them borrow my system to begin with.