They cannot. However, you have to be careful because if she accumulates too much money in her savings or comingles that money with non protected funds, then that is suseptible.
You may also have to file paperwork to protect the money when they do try to levy. If you don't assert a right, sometimes you lose it.
Finally, if she has real property, they may try to put a lien on it.
My last comment is that maybe bankruptcy is a good option for your mother in law so that she does not have to get calls all the time from these collectors. Get a free consultation from an attorney to see what they think. This really depends on her situation.
