I think you need to let her know one month in advance, because for renewal, the documents have to be processed at least one month before contract end so she will eventually know.
For the air tickets, it depends on the season. If high season, better buy the ticket earlier, say one month. Otherwise, one or two weeks in advance should be fine. You can buy through agent or yourself, you may quote the price from both to compare first.
It would be preferrable if you could watch her packing her things.