Let’s not call it a “cheat”.
Hypothetically of course; what would it mean if someone enjoyed a splendid and very non-keto meal, then checked their blood ketones every 6 hours or so, and the blood ketones never got that low, and are back to the typical levels a day later? (By “not that low”, lets say they never went below 0.5 mmol)
Could you assume that you are indeed fat adapted, and such blips once in a while won’t throw you completely out of fat burning mode?
Well it sounds like your friend had a similar reaction I had, I mean a friend of mine had, where I, I mean she, was able to enjoy a special occasion one day and be right back to normal fat burning the next day. Early on with Keto, this wasn’t the case and I, she, was knocked off course for a good 3 days minimum. Then later with several months of adaptation came the ability to go “off plan” and get right back on the next day. What doesn’t work - I can tell you for sure, is to go off plan for several months. That will result in your friend needing to re-start all over again.
Sounds like your friend knows the ropes now! Thank you for sharing your second hand experience 
A week or so ago I gave myself an Oral Glucose ToleranceTest.
I bought one of the drinks that the doctors here in the UK use.
The received “wisdom” is that you need to carb up before doing the test or you will fail.
I didn’t and passed the test… but the most amazing thing was took my blood ketone level that evening and got 1.6!
The drink has 75ml of pure glucose.
Wait, what?
You used the glucose drink or no? Isn’t using the glucose drink considered “carbing up”?
Tell me about this test… I’d like to tell my friend. 
(Yes I will accept an answer of “google it” but it’s nice to hear from a real person)
Did you test again, say, a day later?
The drink for the test is carby but you are supposed to eat “normally” for 2-3 days before the test so your body isn’t “shocked” by the large influx of carbs when not used to it.
Didn’t want to carb up before the test so I didn’t.
It mostly comes down to your level of insulin resistance/sensitivity. Those still IR or with a history of it will snap back less easily than those who are sensitive. Plus it will depend on the level of carbs you have and how often. It is perfectly possibly for a one hit wonder meal to have little impact or to kick you out and take days to get back on track - just depends on the individual.
