I'm unfamiliar with the Beretta 84. However, if the sights are the basic black notch-and-post type, I suggest filing down the rear sight blade with a flat mill file. This should be done gradually, preferably at the firing range so you can see the results immediately. To prevent annoying reflections, the top of the blade should be reblackened afterwards using hobby paint or soot from a lighter; Sharpie marker will do in a pinch.
Although I'm not familiar with the particular pistol, some fixed-sight guns are intentionally manufactured with one of the sights (usually the front) too tall, with the assumption that the owner will file the sight down to match his or her load of choice. OTOH some guns are sighted in with the assumption that the shooter will use a 6 o'clock hold (i.e. aim near the bottom of the target rather than dead center), and some European pistols are sighted to shoot to POA at ranges beyond what is commonly considered practical in the USA, eg. 25m or 50m, which results in a POI that is several inches high at close range.
If the sights are the 3-dot type, or your friend doens't want to irreversibly modify the gun, I suggest training yourselves to use a 6 o'clock hold. Orange stick-on target dots are handy for this.
