You would have to save your CC InDesign document via Save As InDesignCS4 or later (IDML) for your client who uses CS5 to open. There could be reflow issues.
CS5 is no longer distributed through official channels. The best you will be able to do is a trial, which may expire in 7 days (not sure if the change in trial lengths affected old versions), or try to avoid a scammer on the auction sites (you will not be able to register an old version if it was part of an upgrade chain or an educational license). For the trial: Download Adobe Creative Suite 5 products (direct from Adobe) or Download Adobe CS5 Free Trials – All Direct Links without Akamai | ProDesignTools (be sure to follow the directions on the page).
Beyond that, don't use any features on your end not supported by CS5 and save as IDML and your client should be able to open that if they are fully patched, but it's no way to work. Text will reflow.
No. You’ll need to send an IDML.
I’d also send a PDF so they can see the layout because text reflow is very real possibility.