A friend of mine started a company with some others (PTXP) and now it is being bought out by ETE.

PTXP is set up where the stock owners are responsible for taxes at the end of the year (not sure what that structuring is called). Anyway ETE is set to purchase the remaining stock. What usually happens in this scenario and will we still be paying taxes ?

I'm painfully ignorant so any input appreciated

You will not pay taxes after the sale.  Any taxable impact will occur in the year the sale closes -- unless further consideration is paid in subsequent years.