It really bothers me to hear people talk about the boost in alcohol a beer receives from a fruit addition. In most cases fruit won't increase the percent alcohol of a beer, and if you're adding fruit to a strong beer it will actually lower the final alcohol content.
The issue is that many people mistakenly assume that they can determine how much gravity the fruit will add by calculating the amount of sugar it contains. The problem with this method is that in addition to sugar the fruit also contains water (which dilutes the alcohol/sugar already in the beer). In fact, most fruits have a similar sugar content to a standard gravity wort, between 1.040-1.060. There are some exceptions, for example in a couple days I'll be getting 5 gallons of Cabernet Sauvingon grapes (to add to sour beers) that have a sugar content of 22 Brix (1.092). Concentrated and dried fruit are another exception because most of their water content is removed during processing.
To correctly determine the impact on the alcohol content of adding fruit you need to determine the effective brix/plato of the beer. To calculate this you need four pieces of information:
1. Weight of the fruit (not including pits/stems/seeds).
2. Brix/plato of the fruit (from a refractometer or packing info).
3. The original brix/plato of the beer (OG reading).
4. The weight of the beer (based on the current volume, but the density of the wort before fermentation. This can be determined by using the following formula: weight of the beer = original gravity of the beer x volume of the beer x weight of 1 gallon of water)
Effective Brix/Plato = (Weight of beer x Brix/Plato of beer + Weight of fruit x Brix/Plato of fruit) / (Weight of beer + Weight of fruit)
For example if you have 4.5 gallons of 11 P (1.044) beer it would be 1.044*4.5*8.35 = 39.2 lbs of beer onto 10 lbs of 14 P cherries, the effective OG would be (39.2*11+10*14)/(39.2+10) = 11.61 P. That is to say the increase in the effective starting gravity was .61 P, enough to boost the alcohol by .3% ABV assuming the same FG (about the same increase in alcohol from the priming sugar).
On the other hand if you add the same 10 lbs of 14 P cherries into 4.5 gallons of 25 P (1.106) Imperial Stout you would have 1.106*4.5*8.35 = 41.6 lbs the equation would be: (41.6*25+10*14)/(41.6+10) = 22.9 P, a drop of 2.1 P, enough to reduce the alcohol by 1.1% ABV assuming the same FG.
These two examples demonstrates a key insight, if the gravity of the fruit is lower than the original gravity of beer the addition is going to lower the effective original gravity and thus reduce the ABV (and conversely if the gravity is higher it will raise the effective original gravity and similarly the ABV).
In general fruits provide sugars that are more completely fermentable than malted/mashed grains, so even with the same OG you may end up with a lower FG and thus marginally more alcohol. That said, if you determine your effective OG this difference will be included in your ABV calculations when you take the FG reading.
While you can go through all that work to get a slightly more accurate measure of the alcohol in your beer, the main point of this whole rant is that the change in alcohol due to the addition of fruit is small enough that you can comfortably ignore it. Although you might want to pay attention if you are adding fruit to a base beer that is exceptionally strong or weak, or using a large amount of a fruit that has a substantially different gravity than the beer it is being added to.
Sorry about all the math, just a bit of the Mad Economist sneaking out...