selling a tenanted unit

A neighbour is selling their strata unit. They have been unable to sell it for 5 months.

I recently found out that their tenant sits in the common area back yard during inspections and gives negative misleading information to potential buyers regarding strata issues (she does not want the unit to be sold and have to move). She obviously does not disclose she is the tenant. She has even gone as far as getting the phone number of a potential buyer who came over to get a bank valuation on the property, and has been in regular phone contact with this potential buyer.

I would like to tell the owner, but feel I’m stuck between a rock and a hard place as I know both tenant and owner.

Any suggestions on what to do in this instance?