My daughter's '06 Altima would from time to time be sluggish starting, especially in warmer temperature or after it had ran for a while. A couple of weeks ago it wouldn't start at all. Had the battery checked and showed it was bad. She replaced the battery. It has been doing fine but she says it will still be a little "sluggish" like the starter is turning over extra before it starts. I had it checked at auto parts store and their test showed the starter is going bad. My brother-in-law's acquaintance (who works at Nissan dealership) says it is probably not the starter but a fuel system problem and it should be brought in for a diagnostic there. Should I go with results from the free test that showed starter is bad or take it to dealership and pay their diagnostic fees?

Go with the starter. Also, replace the spark plugs, with the same ones that came with the car, if they have not been changed in 30,000 miles.