I would never buy more SP, that really would not be worth my money.
see, you are selfish
you want yourself to be given and not giving back. Better read @raycoms reply
It is definitely worth investing in SP, since it gives you back through curation (which is probably more than what you get at your bank at the moment) and you are able to grow with the price of steem, which might get you some extra $.
how do you know she can afford it ? Arrogant muppet