Review the original item:
Ryan is highly allergic to cats,
he requires at least twenty god-bless-yous every time he visits Mariko
and her seven Siamese.
You wanted to fix it this way:
Ryan is highly allergic to cats he requires
at least twenty god-bless-yous every time he visits Mariko and her seven
Siamese.
To delete the comma between cats and he would cause an equally bad problem, a fused sentence. A fused sentence occurs when you have two main clauses joined with no punctuation whatsoever. Ryan is highly allergic to cats is the first main clause. He requires at least twenty god-bless-yous is the second main clause.
The spot between cats and he needs a stronger break than blank space.
You might want to consult the rules for fixing comma splices and fused sentences.