If one artist leaves the collaboration, can the other artist to continue the work alone?

The remaining artist should acknowledge the other collaborator’s joint authorship in some way.  Strictly speaking, the remaining artist should also secure the collaborator’s agreement for the continuation and development of this joint work; but where the collaborator has unilaterally broken the original collaborative agreement with the remaining artist (and may well owe the latter an element of the expenses incurred in making the art work), the remaining artist may wish to consider proceeding without such further agreement from the collaborator and await contact from that collaborator objecting to this being done.

In this case, the remaining artist should be in a strong bargaining position, and could, for example, insist on receiving the collaborator’s permission for the continuation and development of the joint work in exchange for the artist continuing with the project and releasing the collaborator from a breach of the original agreement and any money owed. Such an agreement would best be secured in writing signed by both artists.