It is understandable, but I agree that it is not quite grammatical. It is a possible slip when you begin your sentence in a very natural way, saying 'this is too obscure to understand' and then you realise you want to add the 'some' part, as an afterthought. But to conceive that sentence as a complete thought - no, not me.
Maybe something like: For some readers this is too obscure to understand.
And maybe a thousand better ways of expressing the idea...
PS. It is wrong because 'by' is normally preceded by a past participle, as in 'understood by', not 'understand by'.
It's grammatically wrong with "by some readers", which requires a passive construction. So your options, as I see them, are:
This is too obscure to understand (which changes the meaning as it implies that no-one can understand it).
This is too obscure to be understood by some readers.
This is too obscure for some readers to understand.
Obviously it should be: "This is too obscure to be understood by some readers." Or: "This is too obscure to understand for some readers." I have no idea why your American friend wants to say it the other way.