The help with her sight is probably Attendance Allowance, which is not means tested & not affected by any other benefits.
If she is getting the guarantee credit element of pension credit (which is almost certainly the case) then once her savings go over £10,000 they add £1 per week to her income for each £500 or part of £500 of savings above the £10K limit. So if she has £10,100 £1 p wk is added to income & that reduces the amount of the pension credit. The credit does not stop - just goes down.
Of course, there's nothing to stop her giving some slightly generous Christmas presents so long as she doesn't go right over the top (too much & she might be thought to have deliberately deprived herself of capital).