Hi, I used to be a Homelessness researcher for Cardiff Council. Depends what you mean by "unit" - I'd have said the local council have a duty of care to house anyone who presents themselves as homeless. It's only when they're "intentionally homeless" i.e. not paid rent on a council house for instance that the Council put them further down the list. In Cardiff there are several hostels but they're pretty harsh environments. There's something like a "6 week rule" - if they can get someone out of the system within that time, they'll be ok, but if not, they get institutionalised and it's harder to get out. I'd say your friend wants to avoid this path at all costs.
I'd say as a woman she is vulnerable, but as has been said before, if there's no room, there's no room. Rough sleeping tends to increase in the summer but in winter, in Cardiff at least, we have a bad weather hostel which is extra provision. A City Centre Team checks on all the rough sleepers in the city and does a soup run and breakfast run.
When she was staying with friends, your friend would have been classed as "hidden homeless". This is a problem that is being addressed, but as there are more urgent cases it always gets pushed down the list.
There might also be a bond board in your town which helps people put down a bond on a flat. The council should be able to tell your friend all this info.
I haven't heard anything about having to stay at the unit all day before they do something though. More likely they were overwhelmed with cases and tried the most plausible excuse to get rid of her.