The photo of Momento was taken 8 months ago. Let's hope it hasn't changed since. Käfer sounds like a good option but it's slightly hidden and it should be easier for the OP's mom to find Momento than Käfer if she is the one who is more likely to get lost. There are two ways (two pedestrian bridges) to the Sheraton which could confuse if you haven't been there before.

Hope it will work out ok. I'll be there in a few weeks but probably not until after the OP's mom.