Upvote:3
There are no fixed requirements about future validity of permanent residence cards for Schengen visa purposes.
Your mother's status in the US is relevant in two ways for a Schengen visa application:
To show that she resides legally in the country where she files her application.
To bolster her claims of ties to the US (because ties to somewhere you don't expect to be allowed to keep living are not worth much for visa purposes).
The first of these is satisfied if the card she presents is valid at the time of application.
For the second, the important factor is not the expiry date on the physical card she presents, but whether she has a reasonable expectation that the US will continue to let her live there indefinitely. If the upcoming green-card renewal is just a routine replacement of the physical card, then there's no reason for the consulate to care about it. On the other hand, if it involves a substantive decision about whether she should be granted a new card, and it's a realistic outcome that she may be denied, then she should probably be more careful to document her reasons to expect that decision will come out in her favor.