Of course you just let them pass! What do you expect the policy to be? Demand a doctor's note to allow guests to pass? The policy is always not to give the guest a hard time, but the purpose of this FP policy is to allow guests to enter if they were delayed. If they wanted guests to just enter whenever they please after a given point, they would save themselves money on ink and make the FP smaller to save money on paper and just write "enter any time after 3 pm."
Yes, because, of course, people are never actually responsible for their actions--it is those in charge who must keep people from doing things out of control, right? I love America. Do whatever you can to git 'er done until someone says "no".
That leaves way too much to the CM's discretion, and suddenly the CM becomes the bad guy. "Oh, I'm sorry...sure, your FP said you need to enter b/t 2 and 3 pm, but in reality, that was a lie--you needed to enter between 2 and 5 pm. I don't care that your grandma died, that's just how it is." They need a blanket policy--either you can always enter after a given time (but still on a given day) or you must return within the listed timeframe. FP was designed, from the start, as a guest control mechanism to prevent the waxing and waning in wait times that used to occur--spread guests out throughout the day so that waits can be manageable, and by having "invisible" guests in the queue line, they can be out eating and in the gift shops (plus, Disney can make smaller, cheaper queues for future attractions). Have an open-ended end time sacrifices that guest control, and it gets more and more out of hand as this loophole becomes more publicized.