Such an guarantee is very unusual from a SIFI. Most investors are nonchalant about hedging the possibility of a US default on its shorter termed Treasury obligations. It sure looks like financial advisors and even portfolio managers haven't explored hedging with hard assets or futures contracts on non-US currencies. Central banks are making appropriate preparations but the mention of increasing dollar swap lines makes me LOL. If people panic and sell Treasuries the first thing they'll do is get rid of US dollar proceeds as fast as possible. Central bank dollar swaps are just as likely to amplify a run on the dollar as mitigate it.
It's too late for your grandma to open a bank account at Chase to seize on this sweet deal. The next few days will reveal whether one was necessary to bail her out.