Hydralazine is a vasodilator, not a water pill (diuretic). Maybe you're thinking of hydrochlorothiazide, which is a diuretic?
The various professional societies seem to have concluded for now that there is no evidence that ACE-inhibitors or ARBs increase risk of Covid-19; there are theoretical arguments going both ways. They recommend not switching away from them, mainly because they think some people will end up with poorly controlled blood pressure which definitely is inadvisable. But this does not apply to your mom who has already switched. I'd suggest having her doctor and your mother do whatever they think best for treating her blood pressure (considering factors such as effectiveness, side effects, ease of taking, cost), without Covid-19 entering the calculation.
The various professional societies seem to have concluded for now that there is no evidence that ACE-inhibitors or ARBs increase risk of Covid-19; there are theoretical arguments going both ways. They recommend not switching away from them, mainly because they think some people will end up with poorly controlled blood pressure which definitely is inadvisable. But this does not apply to your mom who has already switched. I'd suggest having her doctor and your mother do whatever they think best for treating her blood pressure (considering factors such as effectiveness, side effects, ease of taking, cost), without Covid-19 entering the calculation.