That’s a cheap way of getting out of an answer. I really am interested in how anyone would know that their guardian angel finished their rosary for them. I ask this, because I do not believe any man could possibly know more about God and angels, than any other man. If you could explain this, I would appreciate it.
It is taken on faith. It’s tradition. It’s not doctrine or dogma or anything like that and if one chooses not to believe it it’s OK.
With that said, I read a lot. I have spent the last 30 years reading Catholic saint's writings. I also read a lot of history related and not related to the saints. I have picked up great tidbits along the way and that was one of them. It may have come from St. Pio. I add those tidbits to my prayer life.
It is all a matter of faith. If you cannot believe it then it would mean nothing to you.