Key/Mexican lime, I'd plant it in the ground if you can. It will be much easier to care for, as in, water it. In socal that's pretty much all we have to do and fertilize every once in a while. I have had trouble with every bearss lime I've planted, even the current one in my yard doesn't look healthy and yet my key lime trees give me a few hundred pounds of limes per year. Only drawback is the thorns, they're wicked. If you can get past that it is a great tree and very prolific. Also, just in my opinion, dont let the lawn guy take care of the tree, in my experience they like to hack and prune trees to shape rather than let it grow to produce fruit. Just my .02

Evan

Another tip, buy a grafted tree, try and find one on C35 rootstock the fruit will be juicier and it will help semi dwarf the tree and keep it a manageable in size.

I would also suggest Keylime and inground . Keylime has a distinct flavour + persian lime is the default lime in grocery stores ( atleast here ) Keylime is more speciality produce . Persian limes does have more juice n r seedless , but Keylimes have that burst of tangy lime flavour n your mom can make loads of keylime pie :) .

Containers are a poor 2nd choice rather the only choice for our zones , but since u have an option nothing beats the productivity n growth of an inground tree .

I also would vote for Key lime and for NOT letting the lawn guy take care of the tree...in the ground or in a container. I agree that if you put it in the ground for her, the maintenance will be much easier and the tree will be more productive. And don't hesitate to ask for help here...we don't mind!

Keylime, it's a shame you can't get the giant version, and I second c35 rootstock. Seems like a great compromise in all qualities for an in ground tree.

All the above comments are good advice. However, the most important advice has been neglected. That is, free draining potting mix. If you can find the kind with bat guano and coco coir , it is best for growing both variety of limes.

I would say Bearss/Persian limes are easier to grow. And according to several online articles, this is the sentiment. Bearss are the most commonly sold in supermarkets, they have thick skin to protect them from bugs etc, grow bigger and are seedless. Mexican/Key lime have thinner skin, the tree is thorny and they may also have seeds.