Guidelines have changed over the years & the recommended numbers are lower than they used to be.
The guidelines were changed from 140/90 to 130/80 in 2017.
This is precisely why my doctor did not want to prescribe meds for me. In his experience, almost everyone who had elevated readings are now considered to automatically have high blood pressure under the new lowered numbers. So, he wasn't too concerned when I was within that previous borderline range. He did not automatically prescribe meds.
High blood pressure & diabetes run in my family. And I've been (borderline) pre-diabetic for YEARS. There have also been times when my BP was 150/85. So, I have had to really be diligent about changing my diet and trying to get more movement, if not exercise.
I really try to stay within the American Heart Association's sodium intake recommendation of no more than 1,500 milligrams (mg) per day. I'm really good (most days) about eating salt free or low sodium for breakfast & lunch. (Eggs - no salt, peanut butter or nuts - no salt, bread is low sodium, high protein pasta - low sodium, as well as the sauces, any snacks are usually low sodium & lower fat, and lower sugar, etc.
Dinner is where the majority of say 1,000 mgs of sodium goes. But, I salt very carefully. Remember almost every
processed ingredient added will contain salt: butter, canned tuna! Canned corn, canned peas, canned beets, canned chick peas, and canned stocks/broths, unless the label clearly states, "no salt added" or "low sodium." Even condiments have added salt & sugar.
I stand in supermarkets reading and comparing labels all the time. I joined a few Facebook groups which are great about recommending certain brands of food products so I'm reading less labels in supermarkets and only checking to be sure the manufacturer hasn't changed the product & sodium levels. Facebook groups for high blood pressure also have recipe recommendations.
Then, once in a while I can "binge" on a higher sodium dinner of a food I miss, (or am out eating with friends,) which doesn't blow the whole 1,500 mgs of sodium.
A while back, I remember walking around a local street fair last summer. One booth was doing free blood pressure readings with the old style stethoscope readings, instead of a digital monitor. He said my BP reading was a (low for me) 118/75. I was shocked. I went home & checked with my 2 digital monitors, (the second monitor I was given by chance and I kept it,) & the guy was right!

And it stayed around that level.
My last doctor visit a couple months later confirmed the lower levels, as well as my A1C levels for pre-diabetes was down slightly and I lost a few pounds.
So, (I'm NOT an doctor or Internet doctor,) but it IS
possible to manage both if one IS diligent with their diet & exercise. I'm not a saint by any means when it comes to monitoring. I may blow my sodium intake if I'm out and all there is, is a nearby McDonalds. (One burger is most of the 1,500 mgs.) But, I try to make up for that high sodium day by doing almost no salt the following couple of days. I'll eat salads with just olive oil & vinegar. No salty or sugary dressings, no salty croutons, and watch every vegetable I put it, especially if they are canned.
Your DSis has to be honest with herself, how diligent will she be with diet & exercise? Will it be like most people making new year's resolutions and slipping back into old ways within a couple months? Or can and will she really be able to be monitor her diet & exercise more consistently and make some lifestyle changes?
She doesn't have to be perfect every day, IMO, especially at the beginning, until she comes up with a routine that works for her. And even then, she may fall off here and there, as she's human. So no beating herself up when she goes off. Just start again the next day. It's one day and one meal at a time. And luckily her BP levels aren't dangerously high, so she can find what will work most consistently for her. (Again, only my opinion.)
However, she may not have shared with you other medical factors she has that her doctor already factored into her situation, and that is why he's recommending she takes meds.
She may want to consult with a doctor who, like mine, is more lenient on the borderline levels, if diet & exercise changes are made, instead of simply prescribing meds.