The two words 'decent'and 'liberal' are not applicable in VietNam, in most locations.
Each night, around 22.00-22.30H, right across the country, policemen drop in on hotels and pick up a copy of the register and associated documents. If the policeman finds something 'inappropriate' he calls for backup and the unsuspecting guest has a surprise, unannounced visit.
I suggest that you rent a room for your 'friend' - she/he will have to use their identity card in the process.