"He'd heard what she planned"

It’s fine.

“He had heard what she planned.”

Or, something like, he already knew about what she planned.

The use of “had heard” rather than just plain “had” suggests that there was some other event or state not provided in your post, but in the context being referred to which the hearing preceded / i.e. another later, but still in the past, reference point.

The perfect aspect added to a past time verb (had heard) usually sets up a sequence of past events.  Something happened, but something else happened first (had happened).

Typically something like:

 "He'd heard what she planned (before she arrived etc)”