As you do not (presumably) hold a valid disabled exemption certificate, then you are not entitled to use any part of a disabled (free) tax disk. So if you are using or have the vehicle on the public highway, you are already committing an offence - which could lead to the vehicle being seized.
In fact, the scheme rules require that the free disk be returned to the DVLA on the disposal of the vehicle.
Your friend, should have given you the green slip from the 'new' V5 - this would have allowed you to tax the car. As it is, you cannot tax (and therefore have it on the public highway) until you get the new V5 from DVLA.
Get it off the public highway and declare SORN until you get the new V5 and tax it (which automatically lifts SORN).