I am an inventory clerk.
If the inventory doesn't show something, it does not exist. If I miss off a defect or damage to a property, it can rarely be charged for.
Her inventory is apparently correct in that it doesn't state anything in the courtyard.
If an inventory clerk does the make for a property, they ask the landlord what is and is not included in the rental property, so if garden furniture was present that the landlord was not leaving behind it would not be on the inventory, and therefore would not be part of the rental.
Incidentally, if I am conducting a check out and the tenant has left behind items they do not need without asking the permission of the landlord, or agreeing with the new tenant to take them on, there could be a deduction to the deposit for removal.
Putting the issue of the agent not showing prospective tenants around to one side for a moment, the particular letting agent is too much at fault, having only said as far as I know, it's staying as is. He/She was wrong, he/she should have checked, but as the plants are not part of the rental, there is little he can do, short of suggest the new tenant makes a reasonable cash offer to buy the plants from the outgoing tenant.
The real arse in all of this is the new tenant, ONE for not asking in the first place, and TWO for being nasty and threatening about it. Mark my words, that tenant will cause trouble for the LA and the Landlord, it'd be better to refuse them the tenancy if at all possible. They are going to be trouble.
There is no need for this to be a deduction to your friend, if any deductions are made, it needs to go to arbitration and they will look at the inventory at check in, the inventory at check out and clearly see that there is no mention of pots and that they are the property of the tenant.
If in doubt, always got to the CAB, they are AWESOME as are the TDS.