Well you left out some critical information such as his blood pressure starting point, the actual numbers and also any other information about body size/type and history.
IF he were my patient, which he aint, I'd make sure he got his bp under control before he started any AAS.
The reason why blood pressure is raised is because it influences aldosterone, another steroid molecule (cholesterol base rings are steroids including birth control pills). It acts to retain salt in the body, this leads to increased blood pressure.
Now, diovan is a good drug, I usually prescribe altace and take it myself. It is also an ace inhibitor. Which means angiotensin (I,II) inhibitor. Basically another control mediator compound in the kidneys.
Why is an ace inhibitor good? Because the kidney is like a thermostat, some people's are just naturally set high, you take an ace inhibitor to turn down the amount of fluid that is held in the body which relieves fluid pressure in your hydraulic system.
Sort like a release valve, lower the kidney thermostat, lower the fluid and blood pressure.
You can do this in combination with a diuretic and I would recommend a milder one that lasix such as hydroclorothiazide. It works on a different part of the nephronic tube and has more room for error than a loop diuretic such as lasix (less potassium loss).
When you take AAS, it raises the aldosterone which is what angiotensin converting enzyme does a step before, so it basically helps offset the increased level of aldosterone, which leads to less salt retention.
Your buddy can also most likely lower his blood pressure by cutting his salt intake by 1/3. Recent studies have shown that mortality is also decreased by 1/3 when that is does. This lowers blood pressure as well.
Why would you care about bp at all?
Well if you have high blood pressure to begin with, you may be at risk for developing a malignant blood pressure, which is 210/120. Which usually results in death in 3-6months.
It is a rare but serious problem.
Also, with increase of afterload on the heart, the heart is a muscle and responds to steroids as well, it has to lift weights or PUSH against the increase pressure in the arterial system. This leads to a meatball heart, meaning left ventricular hypertrophy. Which leads to several complications, of which I do not feel like typing right now. But the cardiovascular system is my specialty