Soprano and Alto recorders are pretty cheap, relatively speaking. Honestly, it may be worthwhile to explore the possibility of purchasing new instruments. Unlike wood recorders, a plastic recorder's sound will not improve / mature over time, so if the cost of fixing them is comparable to purchasing new ones, it may be a viable alternative.
Personally, if your friend is bent on keeping them, I would recommend cork - the type used on clarinets and oboes. This guarantees a snug fit while allowing for adjustments in intonation, does not hurt the instrument, and can be replaced regularly. It's relatively inexpensive and is easy to apply. Keep in mind that because the cork is designed for clarinets and oboes that it may be too thick for the recorder, so some delicate shaving may need to take place in order to get the cork to the desired thickness.
Also, instruments are made to be adjusted and should never be permanently affixed together - this is a sure-fire way to damage the instrument. I know someone (a music teacher) that welded the slides of his school's brass instruments so that they would never go out of tune. He promptly and rightfully lost his job.