She has the right to assign the remainder of the lease to another party - you do not have to consent to this, but if you do not then she can validly serve notice of termination and she is entitled to leave. It doesn't much matter what the actual lease says, this is one of the statutory rights that can't be overwritten by a lease.
The legislation isn't great on the specifics of this, i.e. if she wanted to assign it to parties totally unsuitable to the property (say a family of 8 to a one bed) do you have grounds to refuse and hold her to it. It just says the right to assign is there.
If the letting agent reckons this other guy is ok, then you may be well advised to consent. Technically, if she is assigning the lease then I'd think it should be seemless, the dwelling will be subject to a tenancy at all times and rent will be paid at all times (it's only a matter of which of the two parties is responsible). The new tenant will come in midway through the lease and the lease will still only run until next July/August.
An alternative and real-world approach, assuming the new tenant is willing to, is to have them sign a new lease and scrap the old tenancy outright.