Why not go for the S7? Sure it's more out of pocket today, but the addition of WATERPROOFING is good for any mom. Plus latest is always the best bet especially since the S7 is 8 months old and that S6 is 1 yr 8 mos old. Don't you love your mom??? Don't old tech her.
But ANY phone will be better than that S4T. Samsung never solved that memory leak issue on the S4 devices. They should have never existed, IMO. Worst phone they've made. On my wife's S6, she complains about the battery life, but quick charging helps that quite a bit. Other than that, a very solid phone still.