Hello, Joanna, and welcome to the Grammar Exchange!
A conditional sentence is a sentence formed by a main clause (what we call the "result") and a conditional clause (what we call the "condition"). Each of those clauses can contain a transitive or an intransitive verb. Only if the verb is transitive in both clauses can the whole sentence be turned into the passive voice, for example:
- If they find him, they'll punish him -> If he's found, he'll be punished.
In your sentence, only the result contains a transitive verb and can therefore be turned into the passive form:
- If she is always late, she shouldn't be waited for. (Although the passive is grammatically correct, the original sentence in the active sounds much more natural to me.)