Co-author illegitimately using results?
Alice and Bob should not worry about it.  Collaboration with Alice and Bob does not prevent Carlos from working on closely related research independently.  Maybe Carlos will even succeed in bringing additional attention to the collaboration with Alice and Bob.

There are two ways this could become a problem:


Alice and Bob insist on coauthorship of Carlos's paper, even though their only contribution was a brief discussion.  They very probably have not earned it.
Carlos does not attribute results from the coauthored papers correctly in his sole-authored work.  Since Alice and Bob apparently have not seen Carlos's paper, it's not time to think about this yet.