Coreg says directly in the drug information that it can cause dizziness, lightheadedness, and fainting. Also, it states that dosage may need to be changed if this occurs. Are her doctors not aware that this may be a direct consequence of the Coreg, or have they just decided that the Coreg is so essential to her treatment that they will have to work around this side effect rather than switching her from Coreg to another drug or changing the dosage of the Coreg?
(If you google for Coreg you can find any number of sites with the prescribing information about the drug. Here's some info right here on MedHelp:
http://www.medhelp.org/drugs/Coreg/show/2905)
I cannot find a drug named "Minitodil." The closest blood pressure drug I found was "Minoxidil:"
http://www.medhelp.org/drugs/minoxidil/show/3811 ... is that it or is it something else? If that is it, again you'll see dizziness, lightheadedness, and fainting right there in the prescribing information.
As a rule, these being drugs that lower blood pressure, those warnings come with the territory. I'm not a cardiologist so I don't know your mother's case and why she needs these drugs. From the sound of it, though, her Orthostatic Intolerance and Syncope (lightheadedness, low blood pressure, and passing out when she stands up) are medication-induced so the first thing to do is to talk to her doctors about these medications and the side-effects they're causing and see if there are any other options.
As halbashes said, you may want to post in the Heart Disease or Heart Rhythm community as well if you have more information about the nature of your mother's condition which led to the prescribing of the two cardio meds in the first place; with knowledge about her condition and the drugs, they may have more insight/suggestions than we've been able to offer here.
Keep us updated if you have time, and let us know if there's anything else we can answer for you.