If the rent is being paid on time and the property is not being damaged, I am not sure why your friend would care. I also find it strange that your friend does not appear to know where his girlfriend lives.
As for the legal issues, unless the lease agreement states that subletting or having roommates is not allowed then the lessee is free to sublet. The lessee remains responsible under the primary lease so even if the subtenants fail to pay the lessee, the lessee must still pay the rent. Also, the lessee remains responsible for any damage caused by the subtenants. Allowing a tenant to sublet has the advantage of keeping a constant rental stream in place the event the lessee has to move prior to the end of the lease. The disadvantage is not knowing who is occupying the premises and the potential mess should the landlord need to evict the lessee. Your friend should review the lease agreement to see if subletting is allowed and consult with an attorney if he is not sure what to do.