If she selects 2012 and contributes $500, she will have contributed the maximum allowable amount for 2012, and has the option of contributing the maximum amount
for 2013 as well (if she chooses to do so). If she does not max out her
contribution for 2012 by April 15, 2013, that opportunity is lost, though
she continues to have the option of contributing the maximum amount for 2013.
Whether contributing the maximum allowable amount to a Roth IRA is a good
thing or bad thing is something about which unreasonable people might differ.
Edit: @JBKing's answer makes a very good point that I will elaborate on
and incorporate into this answer. Eligibility to make a contribution for 2012
is presumably a given at this point (there is at least $5000 in earned income
and AGI is not too large (see Pub 950 for details)), but eligibility to make a
contribution for 2013 is as yet undetermined. If the 2013 AGI turns out to
be more than the limit so that the OP's sister cannot contribute to a Roth
IRA for 2013, that contribution for 2013 made today will have to be
withdrawn or re-characterized as a non-deductible contribution to a Traditional
IRA for 2013. To the best of my knowledge, it will not be possible to
re-characterize it as a Roth IRA contribution for 2012 at that point in time.
So the opportunity to add $500 to the Roth IRA will be irretrievably
lost. As JoeTaxpayer says, why not keep options open by making a full
contribution for 2012 while the opportunity is there? In this sense, I
think there is a downside to choosing 2013 instead of 2012 for that