When your mother-in-law applies for Medicaid medical coverage, the state Medicaid agency will consider the income and assets of her "household." This means the income of her and her spouse, but it does NOT include the income of her adult son who lives with them.
However, anything your mother-in-law and spouse receives from their son in the form of regular payments (including the $300 per month rent) or the equivalent of payments (if he regularly buys food for all three of them, for example) can be counted as income by Medicaid. If their son also regularly pays other considerable amounts (say, the equivalent of more than $500 per month in addition to the $300 per month in rent) for the upkeep of the household, then there's a chance that the total amount -- SSI plus all the payments from their son -- would make her ineligible for Medicaid. This is unlikely, though, and if the only amount your mother-in-law receives from her son is the $300, that probably would not be enough to disqualify her from Medicaid medical coverage, given that their only other income is SSI.