I have had no success with opening V10 drawings in V9 and according to 20-20 tech, not possible. V11 and V10 seem to interchange OK but that will not solve your problem.
You can install V9 on the same machine as V10 without an issue so you could always do this and supply them V9 versions.
We are not rolling out V10 or V11 for full use due to some bugs but a lot of our designers have both and do the design in V9 (because it doesn’t lag) and then do any needed renders in V11 but are careful not to save them back – they usually save them as ‘original file name’-V11.KIT