который никогда не знал его бабущку
"который никогда не знал свою бабушку"

If you say "его бабушку", it makes it sound as though you're talking about the grandmother of a third party -- as in, "Tom never knew Bill's grandmother." Since you're talking about the man's OWN grandmother, use the "reflexive possessive" form свой (which agrees in gender and case with the noun that follows it -- thus, свою бабушку).

Similarly, a bit later in the story:

"[бабушка умерла] 22 года до рождения её внука"
This sounds like you mean "Mary died before the birth of Jane's grandson", and should instead be "...до рождения своего внука." (Which clearly means "before the birth of her own grandson") And, again, своего is in the masculine genitive singular in agreement with внука -- the fact that the subject бабушка is a feminine noun has no relevance to which form of свой should be used.

Also, a couple times you use the spelling девушкы -- however, this should be девушки, because it's a general rule of Russian spelling that the vowel ы can NEVER come after certain consonants, including the letter к.

"...встретился с бабушкой в форме прекрасной девушкы, реалного человека не призрака."
I would suggest, instead:

Бабушка появилась на автобусе в форме прекрасной девушки -- не призраком/привидением, а плотью.

"The grandmother appeared on the bus in the form of a beautiful girl -- not as a ghost (instrumental case), but as flesh."

(The word плоть is often used to mean "flesh" or "body" in poetic and Biblical contexts, and also in expressions that contrast "things of the spirit" with "things of the flesh". However, плоть is NOT used to mean "flesh" in contexts like "a peach has sweet flesh" or "Is the flesh of sharks edible?" -- only when there's an implied comparison of the material body with the non-physical soul.)

"Тоже открыл, что его бабушка поженилась"
Он тоже узнал, что бабушка вышла замуж...

To me, открыть ("to discover") sounds strange here because the context isn't about some big secret or mystery; he simply learned a new fact. So I would suggest узнать instead.

In this context, you could say simply "Он узнал, что бабушка..." and it will be understood that you mean HIS OWN grandma.

And remember that in Russian, the verb "to marry" will be translated differently depending on whether the subject of the verb is a man, or a woman, or the two of them considered collectively:

Дедушка поженился на бабушке. (Grandpa married grandma.)
Бабушка вышла замуж за дедушку. (Grandma married grandpa.)
Они поженились. (They got married.)

(There are other corrections to be made, but as this story seems to be some sort of mathematical riddle, I figured we could talk about the translations of numerical expressions later.)