She rented a business space from me and it's been slow, she just found out there were two failed business there before, did I need to tell her?
You were under no legal obligation to tell the tenant that two business before hers had failed. Generally speaking, the extent of your legal obligation in this regard was to make sure that the purpose for which you allowed her to use the premises is a legal use under the zoning regulations. It is up to the tenant to assess the VIABILITY of her proposed business at that location.
HOWEVER, if you as the landlord could foresee that her business would struggle, then it may have been wise for you to discuss that with the tenant up-front. While this is not a legal obligation, it may have been good business sense on your part. Afterall, if the tenant struggles to the point that they cannot pay you the rent, then you are likely to end up with a bit of a problem too.See question