The heaviness you describe can come from two different areas. The first is a beer that is actually very caloric. There are many (well maybe not many, but they are out there) that are extremely caloric but have a thin body. Those can fill you up.
On the other end, there are beers that are actually very light on calories, but due to their body, people (especially non-beer snobs) perceive them as heavy. Guiness is actually a good example. It has fewer calories than a Budweiser and it actually isnt dense (or heavy at all). That's why a black an tan works: the guiness isnt as dense as the harp or bas you pour it on.
The reason I give the backup, is because I'm not exactly sure how to answer you. The short answer is if the beer is highly caloric (like an imperial) and syruppy you could try adding honey, corn sugar, or table sugar to your next batch in place of some malt. That will dry out your beer to a degree. If you do it in moderation, it won't have a huge effect on flavor profiles. In comparison to this batch, the beer will have less body, but in all likelihood, it will still be on the heavy side.
If is actually is a perceived mouthfeel issue (like the guiness example) there really is nothing you can do while still keeping the round mouthfeel that you want with the style.
Bottom line, the full mouthfeel you get from a stout is supposed to be there. Most people feel that and they think "heavy." dry stouts tend to be less syrupy and as a result, turn people off less.
You could brew a brown ale or a porter (which are less full) with the same rough ingredients that she might like, but those really are different beers.
I say, if you like it as is, keep brewing it. Then, brew something else for others to enjoy.
