Another problem:

BTW, after I'd pretty much agreed to rent this room, she told me it
needed a lot of repair. But none of the problems I've found actually
affect me much. This one affects me a little.

She has hot water everywhere but the bathroom sink. The hot water
comes from the laundry room and it's available at the washing machine
and the kitchen sink and another line goes off to the bathroom, where
it's available at the shower and the bathtub but the sink is the last
outpost on that line, and there is none or next to none. This started
after they did digging outside, on the cold water pipe, of couse, but
if that is the cause, somehow it's only affected her hot water.

The pipes are behind a beautiful tile wall. It's at least 6 feet from
the bathtub to the sink**. The valve under the sink turns easily,
quite a bit from off to on and I'm sure it's all the way on. I guess
I'll turn it on and off a few more times, but .... Is there any point
to taking the valve apart on the hope that the blockage can be pried
to pieces with an awl or something?

The faucet itself is one-piece, one handle, and when all the way to
hot, almost nothing comes out, and it's not hot. Maybe the blockage
is between the valve under the sink and the faucet, but I have no idea
how to take this strange faucet apart. ?? She's short of money or
she wouldn't be renting me the room in the first place, but the rent I
paid her will go fast if she spends it on a plumber, and heck it's
broken. What an opportunity for me.

**Here I was assuming that the problem was only at the sink. Now I
see that the diverter at the bathtub won't allow the shower in the tub
to be turned off completely, and the hot water pressure at the tub is
much lower than the cold water pressure. I've taken a shower, but
even in the US I don't understand the one-handle shower valves so I'm
not sure if the hot water pressure is low there too.

I can get a little more information but this is most of the facts.
What would you do next? What do plumbers do in this situation short
of ripping out the tile and the hot-cold water controls?