The rules are the Platinum member must be a guest at the property, so going by the T&Cs, no, they should not get the Platinum benefits. Now, in reality, it might happen. All depends upon who's working the front desk and the level of training they have had regarding checking ID of guests. Some FDCs have been reported asking the Platinum guest to present themselves to get benefits and failure to do so results in charges.

Have you checked IATA rates that should be available for your Mom? The difference may make the benefits a non-issue.