Helping a housebound relative with a bit of a neglected garden.

Firstly, is it advisable to strim grass on a lawn/edging/borders while it's
wet? It's been raining on and off and it's damp.

Secondly, what's the tactic for badly overgrown grass/weeds? On average it's
about 6 inches high but higher in places. I was thinking of taking it all
down a few inches, then giving it a couple of days to get upright again,
then taking it down to a sensible level. Any point to this, or should I just
go at it and do it in one hit?