If Saline Implants Leak, Can It Be Left in Place?

The danger is that a ruptured saline implant can develop "edges" which can irritate the skin, can be painful, or could be palpated just below the skin surface in thin patients.
The implant shell itself probably poses little significant danger unless it irritates the skin.
Removal of an implant or implants can generally be readily performed under local anesthesia, possibly with light sedation if one of these issues arises.

It is best to take them out however they can be left in if they don't irritate her and lead to complications.

When saline implants leak the fluid gets absorbed by the body and then eventually urinated away.  With the collapse of the implant, folds occur.  Sometimes they can poke the patient from the inside.  This can hurt.  the other thing is that the soft tissue shrinks.  It may make this more difficult to achieve closeness with the other side in the future.  We typically advise removal and replacement if possible.

You mention some concern about anesthesia.  It is a short procedure.  Have your friend talk to her Board Certified Plastic Surgeon.  they can then talk about benefits and risks.