I wouldn't use liquid smoke. . . although I've seen it in some recipes. The better option is to use some smoked malt. You can purchase some peat-smoked malt from any HBS. Use it sparingly as the smoked flavors can get fairly intense.
Addtionally, you can experiment with smoking your own malt. Once, I took some specialty grains - biscuit, I believe - and put them on a wood smoker. It didn't take long for the grains to pick up smoky aromas. I steeped those grains (1/2 lb . . . if I remember right) in a extract recipe for a lighter american style ale.
That has been a number of years ago . . . but it is exactly what your friend is asking for. It was a fairly good beer, but I probably won't do another smoked beer. I prefer smoke flavor on my pork ribs rather than my beer