Your friend needs to be realistic about what she wants... first: how flat the response needs to be, and second: what off-axis behavior she needs.
As far as I know, there are no commercially available dome, compression, bullet, or piezo tweeters that are remotely flat to 50kHz. The ones that advertise extreme extension (45kHz, etc.), usually are 12dB to 18dB down at 50kHz. Not flat at all, and that's
on axis only, off axis the drop off is much more rapid. Ribbons come closest to flat, some being "only" 6dB down at 40kHz or so.
(Despite all the advertising by Sony et. al., no, there are no 1 inch dome tweeters in the world that are flat to 50kHz, not even the expensive Accuton ceramics. Some of the Accuton 3/4 inch diamond domes may do this, but at a raw cost of $2600/tweeter.)
So first off, if her research requires flat response out to 50kHz, she's going to have to invest in expensive measurement gear (standard microphones do not go out that far) and equalization to compensate for the realities of physics. If +/-10dB at 50kHz is good enough, then an appropriate ribbon may satisfy her needs. Some of these would be appropriate:
http://www.madisound.com/cgi-bin/ind...=9914729.28395
The other problem is off-axis behavior. High frequencies are very directional, with quite a drop off even 15 degrees off axis. Obviously, insects are not stationary and fly around. She needs to keep them flying within a 30 degree window from the tweeter in order for them to receive the high frequencies. Ideally, this means the tweeter should be placed far back from the insect cage or housing, and a tweeter be selected that has high enough power handling to compensate for the dropoff due to distance.
Anyway, what she should do is purchase a single reasonably flat commercial speaker, say a single Paradigm Atom, along with a ribbon tweeter with good extension. Put a cap and l-pad on the tweeter to cross it over to the commercial speaker. That's the only way she'll get reasonably close to flat extension out to 50kHz on her budget.