Well, you probably have a few options but you'd need to way up the pros and cons of each:

- Just mention it each time she does it ...she may get irritated, get the hint and move away from you ("You just spat on me ..."); stated in a matter of fact way without showing repulsion; and change the topic

- Tell her you battle to concentrate when she comes to sit near you (which is kinda the truth as she's distracting you)

- Keep breath mints and keep offering her one every time she speaks

- I'm not entirely sure I know the reason why you can't get up and move when she sits there if she seems to have so much freedom to move around - even if you can't get up and entirely move somewhere; just get up and walk away for a minute or two each time she sits down next to you