The correct way to say that is, he sat beside her.
Little Miss Muffet was eating curds and whey when the spider sat beside her.
with
beside
I sat beside my best friend waiting to see the nit nurse at school.
A spider sat down beside Miss Muffet.
The book is beside the door... Stand beside me... That's beside the point...
"I sat beside my friend during the movie." (Beside = next to) "I love pizza, besides, it's quick and easy to make." (Besides = in addition)
Miss Muffet was scared by a spider that sat down beside her while she was eating her curds and whey.
She sat beside her best friend at the concert, enjoying the music together.
Little Miss Muffet sat on a tuffet. Along came a spider and sat down beside her. Frightened Miss Muffet away.
A noun or a verb. 'I sat beside the brook to eat my Sandwiches' (noun). 'I will brook no interference in my affairs' (verb).