I have the old-fashioned cooker, too, but I haven't used it in a long time. I don't believe I have ever pressure-cooked chicken, though. It's been a very long time since I cooked chicken in a crock pot, too. I'm usually either simmering or pan-sauteeing chicken. And the foam thing happens to me every time I simmer. "They" say it happens more with white meat than dark, and DH doesn't like dark meat, so that's what I'm stuck with. Sounds like it would be hard to avoid the foam problem in the pressure cooker, since once it's started, there is no way to access the contents.
Yes, that's really a dilemma. I'm curious to know what you do about it, if anything.
All this got me to thinking--now I know why those little globs of gray leak out of the top of my meatloaves. I knew what the scum was on the chicken, but I had never figured out that stuff on the meatloaf. It's protein. Doesn't make it any prettier, but thanks for bringing this up. And come to think of it, even beans foam. Protein. Makes sense.
If all else fails, maybe wearing disposable gloves will make your MIL feel better about it!