Since they both independently dilate the blood vessels, it is conceivable that together they could lower the
blood pressure too much. So Viagra is usually not recommended for men with
high blood pressure. However, if he has vascular damage which is affecting erections, he should see consult his cardiologist to determine whether Viagra, Cialis, etc. are safe for him.
Unless your partner has physiological factors which inhibit erection, there are other many other avenues to explore.