I have no idea what your father in law's overall financial situation might be, but maybe the first thing to do, just as an experiment, is to pick the lowest share price, (i.e., the most conservative estimate possible), in the fund's existence prior to where your records peter out, and enter that into TurboTax, after entering all the other income and deductions. Since long term capital gains are taxed at 0%, then 15%, then 20%, if it happens that a big chunk of the gain, or even all of it, is taxed at 0% you might decide that no more effort to come up with a basis estimate is really necessary.
