Unless the majority of the pellets are forage based, there ARE no benefits. Horses need forage in order to maintain good gut health. There is simply no way around that fact.
Unless one wants to consider the risk of colic impaction as a benefit
Some years back, a good friend of mine was moving cross-country and it was too much of a risk to haul her mid-20's yr old TWH, so she gave him to someone whom she was sure would give him a good home. They gave him more grain than hay over the first winter and it wasn't long before they called her to tearfully inform her he had passed from colic
Horses were not designed to eat grain, it is we humans who made that decision for them.
If hay is in short supply, for the OP's MIL, or the quality is crummy, Tractor Supply carries a wide array of Standlees hay products in a bag.
Pure timothy pellets, timothy/alfalfa cubes, etc. TSC even sells baled hay in a bag BUT if there's an ingredients label on it, Beware, some of that chopped hay has had molasses added to it Standlee Hay Company - Home
and click on the "Products" button at the top