On Wednesday, October 4, 2017 at 10:14:02 AM UTC-4, Iggy wrote:
replying to Leo123, Iggy wrote:
1st option, IF you have a full 4 to 6-weeks of warm left. Scalp it with the
mower set to its lowest height to remove as much weed, especially any larger
weed leaves. Then, scarify, seed and water.

That is what I would do. Fall is by far the best time to re-seed.
Only question is what kind of other grass is there, weeds, etc.
If he has crap grass, weeds that can't be killed with broadleaf
weedkiller, etc, then it's better to kill everything with glyphosate,
make sure it;s all dead after about a week, then mow short, re-seed.
But it's probably too late in the season now for that, not sure
of the exact climate over there.
It also depends on whether you want a uniform looking lawn or will
tolerate a less uniform look. Make sure to use an appropriate, high
quality grass seed and starter fertilizer. In the Spring deal with
the weeds.