the dave ramsey gift is a nice idea.
$100 really isn't anything. so at least it wasn't more she was asking for.
in the meantime i would consider it a gift in your own head - if she pays it back, great. if not, no big deal. you wouldn't loan her money that you needed, right? if you can do without it, consider it a gift and move on. don't tell her that (she SHOULD pay it back after all) just a little secret you keep.
and next time, tell her no. in fact tell her this is the last time, now. "only" twice in two years is twice too many. unfortunately mom you're right when you say you have to let go. my husband and i went through some rocky periods at first and he asked his mom for money a couple times - i HATED it. it felt so wrong. give her the dave ramsey book (heck sign her up for a class if you can, that would be even better - consider it her christmas gift). TEACH her how to deal with money. i wish my mom had. but luckily we found the dave ramsey financial peace university class and are doing much better. you can't help her by giving her money. you can help her by helping her learn how to deal with HER money better.
honestly, if it was only $100 she was short, and she came running to you, think about it. $100 is just not that much. she couldn't have cut back on groceries, or spending money, for a couple of weeks, to make up for it? she needs to learn how to control her money before the next time she comes to you she's short $1000. it's scary that she came to you over only $100, in a way, you know?