I agree and second the Innokin VV/VW v3.0, Innokin makes great products, and unless you just WANT to spend $100, there are some GREAT deals right now! I just picked up the Innokin iTaste VTR that comes with an excellent gift case for $64 on Ebay, and I just found out today from vaping cheap you can get them for $55 at Vaperev, as well as a lot of great accesories that are on sale as well to go with it! They also had Innokins more expensive mods like the iTase 134 mini for $80! I just checked though, and it looks like their site is temp down, so perhaps the had more traffic then they could handle at those prices! I have the Innokin VV/VW v3.o in pearl/chrome and it's not only beautiful, having a somewhat more feminine feel to it IMHO, especially verses my Cool Fire 2 grenade mod lol, but it works GREAT, is still VERY portable, is squared so it doesn't roll off tables easy, is a passthru so she can vape while it's charging and has a lot of the features more expensive mods cost for a fraction of the cost, as you can find a basic kit for around $35, and the complete kit for another $10 at some places!
If you REALLY want to go out, the Provari Mini in satin chrome is only $159, which Provari is pretty much the "gold standard" of vaping devices for a lot of people, from what I hear. If you don't mind paying more then your $100 budget you can get the full size Provari kit with EVERYTHING she needs in a variety of colors for $212.95 and try looking for a promo code.
I vote for Innokin though, as I have experience with them, and then with the money you have left you can buy her some Kanger Protanks, Aerotanks, some replacement coils AND some of her favorite juice! Just a though, hope you find what you're looking for and that your Mom enjoys her gift and bday, what you're doing is VERY thoughtful!