LOL, no you can't force her to abort.
Get her to admit to someone reliable that she actually got herself pregnant as you say. Otherwise, its her word against yours and she can just say you got her pregnant the old fashioned way. Don't forget the DNA test to make sure its really yours. If it is, you might want to consider fighting her for custody, especially given her unstable situation. Maybe the threat of that would scare her into one. Be careful tho.
Oh, and call Jerry Springer. This would make a great show.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh