Alternatively, if the excess perspiration is a daily thing, it could be a separate common condition called hyperhidrosis, which is easily treatable with prescription antiperspirants in most cases.
As a general rule, you want to take readings for your blood pressure after being seated for at least 15 minutes in a relaxed posture, ideally taken on your left arm (assuming you don't have a contraindication to this such as a left mastectomy - if you don't know what that is, you don't have it) and with your arm resting at about heart-level such as on the arm of an armchair.
Over time, regular exercise should cause both your resting heart rate and your blood pressure to decrease. This is both a direct effect of improved cardiovascular fitness and an indirect effect caused by weight loss.
Bacterial food poisoning is by far the most common type, and while most people will recover without the need for antibiotics, it is a good idea to carry something to treat nausea/vomiting so that you can maintain hydration. Most of these agents are prescription-only, but I do recommend some doxylamine (sold commercially as Unisom) and pyridoxine (Vitamin B6). Give a 10mg dose of each one at the same time, about every 4-6 hours for nausea. It's likely to make you drowsy so don't drive after use. This is a common alternative given to women with morning sickness if they can't take certain prescriptions.
If you are reasonably sure you know the source of the food poisoning and that source is a restaurant or other food vendor, then you are likely the victim of bacterial gastroenteritis. If you can keep down fluids, your best bet is to wait out the illness for 1-2 days and seek care from the nearest urgent care center or emergency room if it doesn't clear after that. GO IMMEDIATELY IF HAVING SEVERE ABDOMINAL PAINS OR IF UNABLE TO KEEP DOWN ANY FLUIDS. If an infant or small child is affected, likewise don't wait as they can get dehydrated faster and may not cooperate with your attempts to hydrate them, and so might need an IV to get their fluids back up.
If you believe the source is something you prepared yourself, especially if it may be from plants or animals you obtained from the wild such as fish or wild game, then you run a risk of more serious illness and should go see a doctor for testing. Most hunters and fishermen know how to properly prepare their meals of this type, but it bears mentioning just the same.
As I said before, prevention is the best policy. If you've correctly cooked and prepared your food or watched someone else do the same then you should be safe. If in doubt, don't eat it.
Aside from that, the main thing you can do is make dietary modifications and maintain a healthy active lifestyle with a healthy weight. In particular, try to moderate your use of alcoholic beverages, sodas and fruit juices. Some people also see a significant improvement but reducing gluten products, but this is a substantial dietary commitment as avoiding gluten is far from easy.
By your description it sounds like it might be cold sores, but there are several other possibilities so it's hard to recommend treatment without first knowing what the problem is. If it is in fact cold sores, then they are preventable with medication.
The most common reasons for edema are (broadly speaking) heart problems, liver problems, kidney problems and malnutrition. Assuming it's not the last one, you need specialized medications and regular checkups for heart problems. For liver and kidney issues you need very careful monitoring and strict avoidance of anything that irritates or damages the affected system.
In short - while a low salt diet may play a role, there's too many possible reasons why your mother might be retaining water to say for sure.