Originally Posted by Tony Stark
If Fox uses Quicksilver and it could be construed in anway that the way they treated the character could harm the potential market value of Avengers 2, Marvel could sue.
Honestly, the could sue because they felt like it and keep it tied up in court, but that's not going to happen. The reality is that Marvel will work with Fox on these properties, and I actually think the joint use of the character could be used to promote Avengers. They could honestly cast the same actor in Avengers, as well as DoFP. They just can't have any direct reference to the other film.
No they couldn't they bought the character when they bought the rights to the franchize and they are legally allowed to do as they will with him.