Who initiates these get gatherings? You or friend? If at your home, I would expect to host, but it does not have to be expensive. If she is over for whatever reason and the reason is not for a meal, and it comes to meal time, if she asks, you say, that would be great, this is what I can contribute, what will be your contribution? If she says, I thought you would get it, "oh, sorry, that will not be possible, sorry, we will have to make it another time then, I will see you to the door now". But, but...."Sally, sorry, but it is not in my budget to subsidize your share. No worries. WE will just plan a meal for another time when you are able to do so".
If it comes up about going out to eat, "Sure, that would be great, but just to let you know, I cannot pay for your share. Do you still want to go? Sure!....ok. Then ask for separate checks right off the bat. Check comes, you pay your share with tip directly to waiter." (I would also make sure Sally or whatever name we are using, that she has her wallet/purse with her before you leave or enter restaurant.
If she balks or says something, "Sally, while I have supported you in many ways since the loss of your husband, I am sorry, but I am not your husband. It is not my job nor my responsibility to continue to support a lifestyle you choose for yourself. if you cannot afford to do something, I completely understand, there are many things we can do to continue our friendship that does not have to cost anything. We can talk on the phone, we can hang for a bit, go for a walk, just get a cup of coffee. If you are looking for more in financial ways, sorry, but that is not going to work for me any longer".