Generally you do a walkthrough with the landlord or estate agent.
It takes ages to record every scratch and issue like a wet bit on a wall, a bit of damaged paint work, flooring scuffed, tap scaly. Recording it in a contract with photos they have already taken printed out for you. If not, Photograph everything and save it with the date and get them to sign it.
They will call things ‘fair condition’ like mine tried to do with a skanky oven. You get away with a lot that is called fair they said as it gives a lot of leeway if you cause minimal damage. But also means when you leave if you left the skanky oven in a skanky condition then they could say well that isn’t fair use and make you pay for it to be cleaned even though it was dirty when you arrived. Works both ways.
Ask for a walkthrough with photos. Or contact a solicitor and they can write a legal agreement inclusive of the photos. They can also redraft a new contract or query for this ‘no better condition
But generally if you are making it into a salon it will always be better than when you arrived at your expense. If you make everything removable and concealable when you’ve left eg counters and chairs and mirrors... then paint back to whatever colour it should be fine. I’d think better is used because better to some is bright blue wall stickers ruining paintwork and painting over upvc windows ruining warranty on things and ‘fixing’ stuff doing bodge jobs. My friend actually thought when he broke a bathroom tile it would be a good idea to fill the hole with cement. Not just get a tile in there. Thick cement. He thought it was a great fix and was really happy with his handiwork
Only damage would be eg water issues, heating but you need to know who’s responsible for those and have relevant insurance.
It sounds worrying but it’s a totally common everyday thing written into all rental agreements.
The solicitor you need would be providing services related to commercial property and conveyancing, just search for one in your area. Some may even deal with it all on your behalf like when buying a house. I don’t know a lot about leasehold but it’s still a type of buying so whether your friend would need a property valuation I’m not sure but they are approx £800. They flag stuff like roof issues/boilers/land/literally anything that can go wrong.
Though if your friend is renewing a lease it does seem unusual that they are starting afresh. Makes me think they want everything your friend has already done to improve it to stay and if they removed anything when leaving they would be in breach of contract.