Ok, so I am not a vegetarian. Rather, I'm kind of meatitarian. I love bbq, cured, brined, grilled, roasted meat. My mother, who is visiting us in Boston for the first time, is vegetarian. Where can we go to eat that we will both have excellent meals? We're located in Arlington, but willing to travel as far south as the North End. Somewhere with offerings that are delicious in the vegetarian area, but also offer real food (do you notice a small prejudice there? It's true, I'm prejudiced against most vegetarian offerings. I apologize to any offended).
Pretty much any of the chef-driven farm-to-table places in Somerville/Cambridge are perfect for this: Bergamot, TW Food, EVOO, Ten Tables, Craigie, Oleana, etc. There is not always a large number of vegetarian choices on the menu, but the food is excellent.
350 Third Street, Cambridge, MA 02143
134 Hampshire St., Cambridge, MA 02139
5 Craigie Circle, Cambridge, MA 02138
All the above are good suggestions: pretty much every chef-driven local restaurant will have decent vegetarian options. My own choice, given your location in Arlington, would be Rendezvous in Cambridge or Bergamot. Check their menus: Rendezvous always has one solid vegetarian entree and 4 or 5 great vegetarian options, including a lovely vegetarian antipasti. And then there's the yummy pig for the rest of us.
I'd focus Italian or Asian--both seem to offer plenty of vegetarian options without eschewing the meat ones. But that hardly narrows the options much.
For Italian, I'd take your mother to the North End simply for the experience (sure, you could go to Gran Gusto, but if it's her first time in Boston...). There are a number of threads with good options. I'd highlight places like Marco, Trattoria Monica, and Pizzeria Regina. She'll have the option of a number of pastas, and there will almost certainly be a hunk of lamb or beef on the menu. Take a look at the menus and past threads to see how those sit with your expectations of cuisine and price.
For Asian stuff, you could do Little Q in Arlington for hot pot. Or you could do Thai from Rod Dee in Porter, Vietnamese from Xinh Xinh in Chinatown, Sichuan from any host of places, Taiwanese from Mulan, etc.
A lot of it depends on what your cuisine, price, and ambiance preferences are...
7 Beach St, Boston, MA 02111
90 Sherman St, Cambridge, MA 02140
East Coast Grill in Cambridge has an amazing vegetable entree, called the 'nightly vegetable experience' or something like that. It includes a variety of grilled vegetables, in addition to a starch such as noodles or rice, and condiments such as quacamole and salsa. It's excellent. They also do great meat and fish dishes, so you should be happy as well. While there arent' multiple choices for the vegetarian, the dish they do offer is really sure to please any devoted vegetable lover, and way better than most vegetarian entrees at otherwise meat-friendly restaurants.