You don't need to be homeless to qualify for any social housing. It's just the reality is the homeless have higher priority.
In this area people have priority points as well as banding. Homeless (among others) would be band A, but there are other properties offered with priority to those in bands B, C or D. There's a longer wait though for those ones.
Age restriction housing is the same here, some will be band A, some for lower bands. Obviously because those younger can't access it, and old people tend to die or go into a care home a lot more quickly than younger people, the age restriction housing becomes available a lot more often so there's less of a wait.
If OP's DF wants housing by the council he shouldn't leave his home without being officially homeless, ie evicted by bailiffs, because if he leaves voluntarily the council has no duty to house him in temporary accommodation until something permanent comes up. In those circumstances he'd be left to stay with friends/relatives/on the streets etc whilst waiting for a council property.
There's nothing to stop him joining the council list before he's homeless if he meets the eligibility criteria, which is usually a "local connection" ie family/job/resident in the area who's list he is applying to join. He'll be graded as whatever (depends on each areas system and his "need" as determined by the council).
Then DF would have to contact the council to update his status in the future, when he received eviction notice and date. Here, this would result in reassessment of his banding and points. OP eviction notice is not the landlord saying "leave", it's the notice of date for bailiffs showing up to make him leave. This happens after its been to court etc. So months away.