What should I use before "happen" "to" or "with"?

[Rachel Adams]

Hello.

Do native speakers use both "with" and "to" before the verb "happen"?

"She couldn't believe the things that happened to/with her."

 

[Rachel Adams]

I'd use only 'to'.

Is it your personal preference or strict grammar rule?

 

[Yankee]

In your example either could be used depending on context/situation, for example:
1. "With": Personality/attitude change following some personal event.
2. "To": Physical or emotional changes following some action/activity.

 

[Charlie Bernstein]

Is it your personal preference or strict grammar rule?

It's not about grammar. It's about meaning.

Words can be grammatical without making sense, and they can make sense without being grammatical.

So we usually say to.