I would consider it legal to use. However, not very efficient. Were I there and knew the situation, I would offer to loan the shooter a gun.
That exact situation happened to me at WR 2015. Had replaced the plunger spring for the slide lock. In pre match testing, it worked great. Worked good first couple of stages. Then slide would not lock back on 50+% of the magazines. I had to make sure it was locked back when moving or staging it. Cost me a lot time locking it back and the mental frustration of it affected my match.