Rights and obligations as a sub letter/lessee in England?

Tl:Dr how does a 'tenant' subletting a single room of the flat they rent to a 'lodger' work in terms of rights and responsibilities? Must deposit paid by lodger to tennant be protected by law? Does the lodger actually have tenancy rights as they live with the tenant rather than the actual landlord?

My friend ('tenant') rents a flat. He sublets (with permission from landlord) one of the rooms to another person ('lodger').

'lodger' was given a month notice to leave (their big dog left alone all day every day causing problems) over two months ago, was given an extension to get past Xmas, and is now supposed to be out by 15th. They are disputing damage their dog has done (low level chewing and scratching on skirting boards etc) being deducted from deposit.

I can vouch that the damage has been done by lodger as I personally decorated the entire flat in 2025 and the damage is primarily in lodgers bedroom.

I can also vouch that any deduction would be of sensible amount, ie half a day's labour for me to go and fill/paint bedroom and hallways to make right for next person.

Is this person a tenant or lodger? Must their deposit be protected? Can they cause a legal problem for my friend 'tenant' and/or my other friend the actual landlord?

Or can my friend just give them back the deposit less a sensible amount for the work and tell lodger to pound sand?