I think you can get her 1994-96 C280. While you are at it throw in a 4matic.
you may be able to get one for about $6K. Nice car with most of MB adored characteristics. This way you get to maintain (MB) it as well as being in her daughter's good graces.. it's a win-win, mon

However, my concern is that rear wheel drive is not as friendly as FWD especially for a senior.