How do you know they signed an NDA and non-compete, or are you just speculating? If so, not sure speculation is helpful here.
As it happens, non-competes tend not to be enforceable in most states, although granted it doesn't say what jurisdiction (or even country) this is occurring in.
Perhaps they are not, but when you have those three agreements signed it becomes hell to try to compete. You shouldn't compete (even if it's not enforceable), you can't talk in any way that reflect wrongly on the other business, and off course you can't share any intellectual property obtained while in the company. I think they are pretty safe regarding competition from the snafu investor.
Most investors won't sign NDAs, let alone non-competes. It's further example of how weird this situation is - on both sides of the table.
You shouldn't compete (even if it's not enforceable)
I would never sign a non-compete because it's not enforcable (I'm in California). If I really wanted to work on a project and the founders said I had to sign the non-compete with no other option I would tell them in writing that I know it is not enforceable and have no interest in adhering to it, and sign it anyway. If the opportunity later came up to compete, I would. Nothing illegal about it.
People go work at rival companies all the time - right now there are loads of good folks leaving Zynga to work for rival companies in exactly the same space - and vice-versa.
As it happens, non-competes tend not to be enforceable in most states, although granted it doesn't say what jurisdiction (or even country) this is occurring in.