She's wrong.
0.9999' is not 1.0.
Both are concepts.
The latter is the concept of 'unity' [ONE]
The former is the concept of the largest number imaginable that is LESS than ONE.
By definition, the former is LESS than the latter.
Never make the mistake of blaming mathematics for being unable to distinguish between the two. The fault lies with the observer and his/her inability to conceptualize.
Infinity is a 'concept'. [Half of infinity is also a concept...it just so happens to be the same number.]
Get your head around this one....
0.99999' is that infinitesimally smaller number than 1.0
0.00000' [with a '1' on the end IF you could somehow reach infinity] is the amount by which it is less than 1.0
Therefore... 0.99999' + 0.00000' = 1
The 'error' arises when you 'think' that means 0.99999' + 0 = 1
Infinitely-recurring decimals, Infinity, Zero, etc are conceptual absolutes.
Just like 'pregnant' is an absolute....there is no 'more' or 'less', just as there is no more or less 'dead'....
Oh, yeah....another pet peeve......'Midnight' is an absolute....you CANNOT have 'half past Midnight'.
You CAN have 'half past 12', but if anything 'half past Midnight' would be 'Midday'....