Sometimes tenants will complain about our service to landlords. As the SM, you should not let this happen. Keeping a good relationship with tenants is also your responsibility. If you don't and tenants reach out to owners for every small thing, then it defeats the purpose of property management.
In this case, do not argue with the owner. Please understand the issue entirely. Tell them you will check with the tenant. Try and understand the issue from the tenant and resolve.
Try to explain to tenant to raise any issues with you first. If you don't respond then they can reach out to owner.
Also explain to owner that we have a professional way of handling tenants. It is better they leave this communication with tenants to us. Otherwise, tenants will keep calling them for small things and asking for favours. We have our own way of handling such requests.