I share your pain. I had a milk frog that was sick when I bought it (didn't check properly that it's mouth was a bit deformed too from not enough calcium intake) and had rectal prolapse like every week. It died after a year of me keeping it. What you can do is feed it smaller crickets, I noticed mine could not digest large ones and would poop the whole cricket out and prolapse would be more severe if the cricket was large.

Make sure the prolapsed intestines are soaked in water until it sucks back it. If it dries out, it'll be a problem. You can try putting the frog in light sugar and water solution (warm) to help it suck in.

A vet will unlikely be able to help. But, if the frog has prolapse once, very likely it will continue to have prolapse rest of life until it passes away. As you just got the frog, probably the breeder/ shop keeper did not feed the frog properly (dusting crickets with D3 calcium), so it has this condition.