It seems that Airbus aren't taking on any of the debt. Which is very nice for Airbus. What Airbus brings to the program is more "certainty"; that should result in lower parts prices, an even more solid sales effort, package deals with other Airbus aircraft, you name it.
And most importantly, Airbus brings manufacturing capacity (in the US too, which now might turn out to be useful). There's no point selling thousands of C Series and securing cheaper parts deals if they cannot be assembled.
Airbus themselves have effectively said that their intent is to go big with the C Series around the world. The opening of an assembly line in Mobile was decided before the imposition of the trade tarif; they simply want to make lots more C Series than Bombardier could manage all by themselves. If that all works, the C Series pie will be a lot bigger than originally planned and Bombardier's slice of it should be good for them.