War Commander: Rogue Assault does not support multiple bases on the same device; if you load your base on your friend's device, you will permanently associate your base with their device. This means that if your friend had a base on that device, it is now gone (there will be a pop up warning about this). Additionally, if they wanted to have a base of their own, they no longer can.
As such, we recommend only playing on your own devices and to "factory reset" your old device before you sell/give it to someone else to avoid a situation like the above.