You as the landlord need to handle this. Check out the complaint by verifying that the upstairs tenant is making too much noise. If true, then issue a cure notice to upstairs tenant to cure the problem. Your downstairs tenant has the right to 'Quite Enjoyment' of his rental as provided in every lease. Quiet enjoyment is a right to the undisturbed use and enjoyment of real property by a tenant or landowner.
Courts read a covenant of quiet enjoyment between the Landlord and Tenant into every rental agreement, or tenancy. Thus a renter, or tenant, has the right to quiet enjoyment of the leased premises regardless of whether the rental agreement contains such a covenant.
In the covenant of quiet enjoyment, the landlord promises that during the term of the tenancy no one will disturb the tenant in the tenant's use and enjoyment of the premises. Quiet enjoyment includes the right to exclude others from the premises, and the right to peace and quiet. Why would you think that this is between tenants, when it is your legal obigation to uphold all lease terms including the covenant of quiet enjoyment. It's your responsibility as the landlord, handle this in a professional manner before this escalates into a bigger problem and a vacant rental.