On a knee, or any other major joint, it is typically better than stitches. Stitches have no give, and will tear out, making the wound worse than it was before. Where-as glue ussually allows the elasticty of the skin to absorb the flexing and stretching. But, it sounds like there was additional info we may not be aware of. Like for instance, too little glue used, bad batch of glue, a chemical reaction between the gleu, and possible soaps used to wash himself afterward, the typical reaction of guys to continually push the limt, and many other things like that.
I would suggest against crying malpractice until all details are acounted for, and another doctor is consulted.