There is no 'security' issue as both parties present their bags and answer the security questions for their own bags, it is simply a 'pooling' of the allowances.
This is still a security risk, as all that has to happen is that passenger 1 does a no show at the gate, and his/her baggage is pulled from the flight - The second passenger boards with passenger 1's baggage on her PNR which remains on the aircraft.
For this reason pooling of strangers baggage is usually frowned upon.
She should work for Ryanair - and probably be sacked if she doesnt collect excess baggage fees.