Funny, that is one reason I took over the cooking of enchiladas in our house - my wife's always fall apart! What I did was heated the corn tortillas (I think Mission brand) for 20 seconds a side on a hot skillet with some vegetable oil before adding the cheese/onions/peppers. Then when I baked them I only used a minimal amount of sauce, 10 minutes at 225C (450F or so). After they came out of the oven I put the remaining sauce on. They were very firm. I think what she does wrong is drenches them with way too much sauce in the oven.