The correct verb is were. If my friend were to lose her job, she would probably not find a new one quickly.
Let's interprete the sentence given;
We are using the subjunctive mood, which expresses hypothetical or unreal situations.
The subject of the sentence is singular ("my friend").
The verb "were" agrees with the singular subject in the subjunctive mood.
Therefore, the correct sentence is: If my friend were to lose her job, she would probably not find a new one quickly.