if they are entitled to legal aid and win, and get some money, they will be requested to pay the legal aid money back.
who were the executers of the will? if they have distributed the estate wrongly, it is indeed very serious. Hwever, you use of "contesting" the will seems to imply that it wasn't distributed wrongly, just that your friend dosent agree with what was in the will? You dont have to be a relative to be a beneficiary. there are usually only very few grounds for contestng a will, and it's an expensive business - for example any benefit might be taken up by the solicitors fees. even if legal aid is udesd, as i said if there is money involvd they will usually be asked to pay it back.
What might be an idea is to get a copy of the wil frist and see what it actually says. If it says "all to (sister)" then the dont have much chance