I think LazyDaisy's suggestion is the right approach.
I also think, given that you say Lisa is generally a nice person, that it may be worth trying to speak to her before the next meeting, at a time when she isn't trying to get you to help *right then* and say something along the lines of
"I've been wanting to speak to you about asking me for help. I know that you have been stressed; I think that's true of a lot of us since staff numbers have reduced. I've been reassigned and am now responsible for ABC, which is very time consuming, especially as I have taken on extra responsibilities so have a heavier workload. I don't have spare capacity to be able to help you with your job as well as doing my own. If you are struggling to cope, I would suggest that in the first place you ask [name of person who is in her department, if there is anyone] and if they are not able to help that you speak to [her supervisor] to let them know that you are not able to cope with your current workload, so they can work with you to address the problem.
I would appreciate it if you could not ask me for help at meetings, as you have been doing. It's not appropriate, as helping you with your job is not part of my job, and you should be making those requests to [her supervisor] not to me. "
If she tries to argue or to critisize you for not helping,you could then add
"I don't want to embarrass you in front of [boss] but if you raise it again in meetings, I will explain that I have already spoken to you, and to [your supervisor] who has confirmed to me that I must prioritise my own job, and am not expected to take responsibility for your job as well"