At the risk of being flamed, I think that this is one of those cases where one must recognize that veganism is not about labels or purity but about saving animals. If you manage to convert one single person to veganism -- even if you just convince them to cut their meat intake in half -- you will save more animals than you would by refusing your boyfriend's grandmother's cooking. You could try to undo the damage of eating at this woman's house by ordering a hundred Vegan Outreach booklets each time you eat there and passing them out to students at a college campus near you. Or donate to an animal sanctuary or to Mercy for Animals. Or plant native plants in your garden, if you have a garden. Or change one habit you have that is harmful to the environment, such as flying when you could take the train, or buying things wrapped in plastic (plastic never biodegrades and will continue to harm animals for millennia after we're gone). Another way to help animals would be to go out in your neighborhood for an hour or two each time you are unable to be 100% vegan and pick up plastic trash, which will otherwise end up in the trees and bushes or in the ocean and harm animals for a very long time.
As you mentioned, you could eat vegetarian when you're with her and vegan at other times. Even if she makes something with meat and you just eat the sides of vegetables, you will still be saving animals because she will cook less meat, knowing that you won't eat any of it. I don't think a compromise like this makes one non-vegan. It's impossible to be 100% vegan in a world that exploits animals so extensively. It would be more productive for vegans to work to turn the world vegan than be obsessed with purity according to the 1949 definition, which, by the way, says nothing about plastics.
That being said, sometimes people can surprise you. Some people are very respectful of other people's dietary choices. But you're probably right to want to tread carefully.