I would model this in excel, but if she sticks to the 4k/mo and invests the other 5k/mo, I would imagine she will be doing just fine.
I would say that by 2029 when she is almost 80, she will probably want something around 40/60, and with such a short time for the money to be saved until then, compounding does not have enough time to make higher risk than that beneficial.
In the best case it's not significantly more and won't change her life.
In the worst case, a higher equity proportion will have less safety margin in term of survival in a downturn and could ruin her last years.
Why take the extra risk and return if when it works out with a lower allocation, it doesn't matter.
It makes no sense to gamble on something that if even when you win, it does not matter. You are guaranteed to lose.
So yea I would allocate anything over the 4k/mo into a 40/60 portfolio. Preferably a fund that is auto rebalanced for you so there is nothing to do except dump it in each month. Vanguard Target Retirement 2015 looks good to me.
Seriously though, I urge people to do the numbers in excel to see how utterly useless a higher equity allocation is for this specific scenario. The results will surprise you. Actually, here I knocked one up quickly. I would never advise my mother to take a 70/30 portfolio with the massively higher risk, when the reward is so tiny.