I've had a Guest 2 check-in many times. Guest 2 has always been my wife, and in physical possession of a credit card from the same account as the one I used to make the reservation. Never been an issue for us... I'm usually, but not always, eventually there - but even when I am I sometimes don't interact with the front desk. We've had lots of stays where we arrive from different cities, including one about 3 weeks ago.

If Guest 2 were a random buddy with no credit card of his own, I can theoretically see that being a problem. Haven't tried it.

Once all hotels have digital key, this will eventually get easier...provided you trust Guest 2 to log into your hotel chain's mobile app as you on their phone.