You could take your student home and shower with him. (JK)
I asked this very question when I gave CFI checkrides.
Your student may not be aware that they smell like Aunt Edna's cat boxes so you do not want to come right out and say "You Stink" (unless it's not your student and you don't care).
Ask what they do to before they arrive at the airport. Suggest that they wake up 30 minutes earlier, shower, get a good breakfast and brush their teeth before coming to fly.
Say that if they show up for a flight and thier body is not ready to fly, they may be wasting their time and money.
Open all air vents.
Maybe try flying later in the day.
You could also try gum or hang one of those car freshener things from the students ear.
I also like the idea of getting very stinky yourself so that your that your natural perfume drowns out the students! This may have a bad effect on your social life. Wait, do instructors have good social lifes???
If they don't take the subtle hints, then maybe sit down with them and address the problem in a more imformative way rather than saying, go home stinky!
In a bigger plane, don your O2 Mask and establish crew communication!
-Night_Flight-