Extreme fire danger and mowing the lawn?

Well it definitely could start a fire, mainly if he was to hit a rock or something that could spark, I probably wouldn’t be mowing if I were him

Possssssibly?

Sparks from the mower
Gasoline
Smoking a cig while mowing (It happens)

When it’s super dry ... it can spontaneously combust in the hot sun

U Is So HoTz U CaUseZ FiRz

Have buckets of extra water available. Good to have numerous amounts. Maybe it won't save ya, but hey, maybe it also could!
Also good for it the lectrick goes oo't.
I'd let him mow his lawn though, or else he might snap or something, having tall grass.

They are worried about the blade hitting debris and sparking a fire

Is he using a gas or electric lawnmower?

Cut down would be safer

Not very common but can happen if the spark arrestor is taken off of the exhaust. Which they almost always are since they clog up faster than a rabbit late for a date.