"Could": past and conditional question

Solution 1:

Is Ben talking about the past? Or is he offering his grandma for tomorrow?

I deduce the following from your questions:

1. If Ben was talking about the past, then the sentence would accompany an earlier date, and would've changed to Oh. My grandma could swim back in the 80s [or whenever she could swim].
2. To answer your second question, I've noticed that there is could and not would. Hence, the sentence remains incomplete, because there also is a why to answer because the could needs it. Now, most speakers terminate the sentence at this point, because it is understood that the speaker might be asking for the permission. A fully-constructed sentence would be Oh. My grandma could swim, if you would/she could...