I second calling Shelter- they have a Legal Aid contract and providing he qualifies financially for LA, he'll be able to get ongoing advice and representation from one of their advisers if they have capacity to take the case on. They're great though, and will seek somewhere else out to advise him if not. This will be particularly helpful if he make a homeless application, as local authorities are notoriously good at 'gatekeeping' and refusing assistance even where there would be a legal duty to re-house.
However, I hope I can offer some reassurance to you about this (I'm a legal adviser working specifically in this area of law, though not for Shelter).
Based on what you've described, he'll have an assured shorthold tenancy and you've already said he's on a rolling contract. The landlord must show that they have served him with two month's written notice of their intention to seek possession. This notice will be valid providing they have given at least two months notice and (depending on whether he moved in after 2007 or not) they protected any deposit paid in a government- backed scheme and served him with the basic prescribed information about where the deposit is registered etc within 30 days of receiving the money. No need any longer, on a rolling contract, for the notice seeking possession to end on a specific day of the month (after a case in 2013 changed this).
Landlord must get an outright order for possession from the county court, then a Warrant Of Possession (bailiff's warrant) in order to lawfully seek possession. All applications will be considered by a District Judge, either on paper or at a hearing, and unsurprisingly, if they notice any problems they're likely to dismiss outright, or arrange a hearing on the matter. Better if your neighbour has a legal adviser from Shelter to pick up on any problems nice and early though- can't always rely on the court, and mistakes do sometimes get overlooked, particularly on paper applications for possession.
Realistically, the landlord could accrue legal fees of £2000 but this is SO unlikely. For fees that high in a straight forward case like this, they would have to be hiring a barrister. It is a District Judge who decides whether the fees proposed by the claimant landlord are 'reasonable' and hiring a barrister, or even a senior solicitor, for this type of case would likely be considered completely disproportionate in the circumstances, so would be disallowed, not necessarily in full though.
If, however, there was a potential defence to the proceedings (for example, the notice is invalid, the deposit wasn't protected) then costs could be higher as there'd be at least one hearing at court. This doesn't mean he'd end up paying them all though. Shelter would be able to give advice upon consideration of his circumstances as to any likely defence, and could undertake court work if there was. They could dispute excessive legal costs.
If a landlord hires a solicitor to undertake the work for them, you'd usually expect to see the average possession case like this costing around £400- £600. He can request the court allows him to pay it by installment, but this will means he gets a CCJ, which will affect his credit rating, against his name.
The reason the landlord hasn't accepted rent (they've definitely had legal advice!) is because they don't want any potential defence raised that they've accepted money and inadvertently created a new tenancy for him. They can charge 'mesne (pronounced 'mean') profits' which would secure them on this though- a way to get their rent money from him without the argument re: creating another tenancy.
More than happy to give you further housing advice if you would like- but he must get proper advice. It is so important in these cases to have a legal professional consider his specific circumstances- I can only ever be general.