Can I rent in a shared 1 bedroom flat ?

My friend who owns a 1 bed flat is unemployed, but said I may be able to rent her room and have use of the flat. She would sleep in lounge on bed settee.

Unsure if this is allowed as only having one bedroom ?

Also if it is, any idea of how much I could get, midlands area.

Also female, in case you thought a couple.

I know they would probably just deduct the money from her benefit, but shes fine with that, to help me out.

Thank you for info in advance.