My Calgary, Alberta tenant did not use the bathroom exhaust fan as she did not wish to clean the vent and now the bathroom ceiling is black with mildew. Should she be made to pay for this. The condo had none of this when she rented it

Perhaps, if you can find an expert to testify that, but for the failure to use the fan, the mold would not have occurred.

But I doubt that you will win this one, particularly if the lease didn't require the tenant to use the fan.

I doubt it too.
Buy a long handled scrub brush/broom. Put 2 tee shirts over your head and then a plastic bag, with a cut out to see and breathe. Bleach twice, with plenty of ventilation. Don't use ANY other product at the same time. Don't waste your money on one of those ridiculous mold remediation companies.

Then hire an electrician to wire the fan to the light switch.

Were you in a flood zone? If you were did you clean your condo according to how you are supposed to. So that you had no mold? I too live in Calgary and helped with the flood cleanup and do know there were specific things to be done.
Some people did not and are just now seeing the side effects from not being through