Can collagen lip injections be dissolved?

If the lip filler referred to has been done within the past few years, it's unlikely that it contained collagen.  Most lip fillers on the market today contain hyaluronic acid (Juvederm, Restylane, Belotero, etc.).  These newer fillers can be dissolved with an injection of a substance called hyaluronidase.  Hope this is helpful.

Lip enhancement and correction are most frequently done with hyaluronic acid fillers such as Volbella, Restylane Silk, and Juvederm. Collagen, bovine followed human, was used for lips until the hyaluronic acids were developed.  Bovine collagen is a part of a product called Bellafill, but it is not indicated for lips.  
Collagen in the lips is not long lasting, approximately 6 months, and cannot be dissolved. 
Hyaluronic acid can be dissolved with an enzyme called hyaluronidase so it is a great product to use in your lips in case you need correction after the treatment, or in case you just do not like the change.

Collagen injections typically dissolve on their own over a few months time.  I have not used collagen in 10 years and do not know who uses it anymore.  There are many more fillers that are better on the market today.