It means the summer that just finished. If she said "last summer", depending on the time of year it might sound like the summer the year before. But this past summer makes it clear that it is the most recent one. For instance, if she said "last summer" and it is already the following year (like January) it would be clear it was the summer immediately previous, but if she said "last summer" and it is only September (i.e. North America/Europe), it would not be clear if it was the one that just past (the June - August just before) or a full year before, so it's clearer to say "this past summer" to indicate it is the one that JUST passed.

This past summer means the summer that just past. For example, if I am talking about this past summer today--I mean summer 2017.

这星期一早上的前一夏季