Check the lease. There is usually a 'Rights' section near to the top which covers how much notice the landlord needs to give.
If they won't allow access in accordance to the lease, they are in breach of terms and in effect, the LL could evict them if they want.
Most leases allow Landlords to have access with a set time frame of notice - if not, I would mention it to your friend and when their tenancy expires or breaks, update the lease for the renewal or new tenants lease to state that the LL has to give only 24 hrs notice before access - this way, you can ensure nothing dodgy is going on.
Your friend might also want to consider making the next lease on their property an internal & external 'full repairing' lease which would make the tenants liable for any repair works during their tenancy - if they go ahead with this, make sure you include a schedule of dilaps in the lease, so that when the lease ends, the tenant is liable for dilapidations costs for any repairs required when they vacate.
A property inspection only takes up to an hour, so why they would want to wait is a quandry.... unless they have something to hide