markobr
Noone has done the embarrassing thing: Nobody ever?
No. Or just nobody of much relevance in the situation?
Yes. Have there been any people who actually did the embarrassing thing anywhere, ever?
Oh definitely. If so: Only people (geographically) far away of Mandy? Only people living way before Mandy? Only people socially far away of Mandy?
No/irr.Is Mandy actually H?A?F? (From the answers we know, it surely looks like that, bun well, you never know...)
Yes. 
Is the relevant thing embarrassing because it is percieved as: immoral? stupid?
This is closest. unnecessary? Contrary to what Mandy usually would value? Illegal? Simply a waste of time?
trebor
Is Mandy out with friends having a drink?
No. Do they have a rule like whoever touches their phone first foots the bill for everyone?
No, interesting thought. Is talking on the phone relevant? Checking the time? Paying for something?
No to all. Did she tell her friends she had already done whatever the thing was with the phone, when in fact she hadn't?
Yes. Perhaps one of her friends was like, "Oh, did you tell [mutual friend] that our plans changed and we're meeting here now?" and Mandy had to lie about it for some reason?
Nothing like this. wunderland
Is the 'action' with the phone: buying it? selling it? lending it to someone? touching it? looking at it? ignoring it? carrying it? leaving it behind somewhere? losing it?
Sort of/nearly. damaging it?
YES. Is the 'action' with the phone the embarrassing thing that she pretends to have done?
Yes. or part of it? Or is it something she actually does?
No.