You inspired many ideas though, and I'll gladly share those back with you.
1. The blonde is large-chested enough to "bust" her friend now.
2. You could also have her try the ray on her own chest, leaving both oversized in front, undersized for the rest of them, and the ray sitting in between where neither can reach.
3a. You could introduce a third character that finds them like this.
3b. A third character could use the ray and have her chest large enough to "bust" both of these two at the same time.
4. The now smaller character could try climbing in between the blonde's cleavage.
5. She could be inspecting the now oversized ray gun and find there's something wrong with it (loose wires, dead battery, etc.)
6. She could be using a giant camera phone to take a picture of her busty miniature friend, stepping on the buttons to activate them.
7. She could shrink the blonde again, but not her chest this time, then the next scene could have her "busting" the blonde on top of the blonde's own bust.
8. She could shrink further, then scale the blonde's bust like a perverse mountain.
9. She could go around behind the blonde's bust and hug her friend, implying she was just having fun.
10. She could return her blonde friend to normal size, then get "busted" herself, or get picked up and carried between them.