"He is therefore wondering if she can pull out at this late stage due to the delay in closing the sale?? "
So - he doesn't have a signed contract. This is the wording we use when dealing with contracts in the preliminary exchanges:
"Please note that we do not have any authority either express or implied to bind our clients in any way and we must emphasise that neither this correspondence nor any negotiations had between our clients shall constitute a note or memorandum within the meaning of the Statute of Frauds until such time as both contracts are signed, exchanged and a full deposit is paid."
Technically, and in the absence of any other information , she could pull out.
However, practically speaking, where is Vendor going to find another purchaser who can complete as quickly as your friend?
mf