How about that black widows are nowhere near as willing to bite as many T's out there. If you keep any asian or african terrestrials or arboreals, then those are much faster with much bigger fangs. A pokie is probably a much bigger worry than a widow. Never kept one, but I believe widows are generally much slower than T's and are much calmer. IF it ever ran out of its enclosure, it would be much easier to recover than, lets say, a Psalmo. Just make it seem like widows are a piece of cake compared to T's. Given, I think their venom can do more damage, they are much easier to handle than most arboreal tarantulas and even some terrestrial.
As far as venom goes there's antivenin and their bites are one of those dangerous for at risk groups only (elderly, young children, sick, etc). If she feels strongly about it I'd just drop it. I can completely understand not wanting a dangerous spider with such a reputation in my house.
I've kept black widows for years - in fact, my very first pet spider was a gorgeous BBW who helped me overcome a lifelong fear of spiders in general and widows in particular. I've also kept brown recluse spiders. Both are timid spiders and unlikely to bite or escape. They are much easier to deal with than some of my tarantulas! Perhaps you can address it from that angle, letting her know that widows are less likely to attempt an escape - and easier to keep secured - than tarantulas. Widows can be kept in a very simple, inexpensive, yet virtually escape-proof container like a 2-liter plastic water or soda bottle with a screw-top lid. They don't even need ventilation as long as you don't put anything wet in there that might create mold problems.
A very basic widow setup: Drop several dry sticks into a clear 2-liter soda bottle as anchor points for her web. Branching sticks are best, as long as they are flexible enough to fit through the opening. I like to take a small piece of very thin, sheer fabric or even a bit of Kleenex and stretch it across the mouth of the bottle, underneath the cap. That way, if the spider decides that inside the cap is the perfect spot to hide (they can and will do this sometimes!) you can remove the cap and poke the spider (through the fabric or tissue) with a pencil or chopstick or something to encourage her to move back down. Otherwise, if you need to remove the lid, you might inadvertently end up with the both cap and spider in your hand when opening the bottle for a feeding.
Widows have very minimal needs. Western Black Widows (Latrodectus hesperus) are well adapted to desert living, so they don't need water - they get all their moisture from their prey. I usually fed mine one cricket every week or two. They also don't need a great deal of humidity or ventilation. The air exchange during feedings is adequate without any ventilation holes in the bottle at all.
You can poke small air holes in the bottle - or other container of choice - if you want to, but that can be problematic if you end up with babies because widow hatchlings are very small and can easily slip through most ventilation holes. If you have a wild-caught mature female, there is a very good chance that she will have already mated, and if she has mated, she can continue to produce viable egg sacs for up to a year afterward. I believe I've had as many as eight or nine viable sacs from some of my widows, and the last thing you or your girlfriend wants is several hundred baby widows getting out and infesting your home! Alternatively, you can use long tongs to pull any egg sacs as soon as they are laid and dispose of them to prevent hatching.
One thing I have noticed with my widows is that they will frequently develop a feeding response where they learn that the lid being opened means food is coming. Instead of running away from the opening, as they will initially do, they learn to run toward the lid and wait right in the opening for the tasty cricket. It's kind of cool - but you do want to keep your fingers back from the opening when feeding them, lest your fingertip be mistaken for a particularly plump, juicy cricket.
Widows are fascinating spiders to watch and quite lovely. Good luck!
Well there is also trust involved. My parents make it obvious that they don't like my collection but they still let me keep hot species because they trust in me to keep them in their enclosures. Plus black widows already live in the U.S. and are apparently common in homes (I say apparently because they are pretty much nonexistent in my area).
Try keeping a plain old house spider. They are pretty much the same as far as the important points are concerned. Once comfortable with the harmless one, work into the Latro's. I've kept widows for many years and never had a problem. Not the first sign of aggression at all.