It's OK to thank someone's guardian angel

But don't worship them or pray to them. However if you are seeking a way to express gratitude for a miraculous rescue, recovery, and so forth, you can thank the guardian angel of your loved one too, in addition to thanking God. It's a way for you to remember that they are always there, doing the best they can with human made conditions and circumstances. And they like that you remember them only because by doing so you are acknowledging one of God's great gifts to humanity through their witness.

And it's a nice personal touch at an emotional time, to remember that your loved one has a guardian angel right next to him or her at all times, who can sometimes "work the angles" of a situation if they can, God willing and if circumstances allow. Guardian angels mostly work to prevent the situation by being the voice of grace and good decision making in one's ear.