1) He has friends on Steam and would like to chat or whatever with them while on Kong
Then get his Steam friends on Kong and chat. Alternatively, have two windows open (unless, of course, you have Windows 8, in which case you’re objectively an idiot).
2) He has some money on Steam and would like to purchase some Kreds
Not that there’s ever a good reason to buy Kreds. Only valid reason I can think of off the top of my head is to buy VVVVVV, but you can do that on Steam already.
3) The underpants gnomes told him to
…Oh. That’s a very reasonable explanation. Clearly, we need to link Kong and Steam.