transmission is blowing fluid out of filler tube when trying to check it on 2000 Toyota Tundra

had a friend bring this to me and I havent seen anything lie it. He had fluid checked at a lube shop and I actually took 1 qt out and then put it back in because it is blowing fluid into and out of fill tube. Hard to get an accurate reading. Trans. is slipping too. Filter plugged??? Bars trans. additive??? help

Asked by for the 2000 Toyota Tundra