It's free to create and hold a SkyMiles account, so who cares if she doesn't fly often. She will need to be logged in to buy the membership anyway. I just tried to look at SkyClub pricing, and it wouldn't let me see prices or purchase unless I was logged in. You can make one for her if you have all of her information. I know many people who create and manage SkyMiles accounts for their family members.

Create a SkyMiles account for her with all of her relevant information. You should use an email address that you have access to or use your own email (not the same one as your personal SM account). Then log in to her account and purchase the 30 day membership. You should be able to print out a temporary card for her and then mail it to her or send an electronic copy that she can print on her own if she is capable.

Bring a copy of the temporary card with you, in case she happens to forget it. Without it, she should be in the computer system but who knows is a recently purchased membership would show up within a few days. It also sounds like when you get to the SkyClub in person, you can get the physical 30-day membership card. You need that to get into partner lounges, but the temporary printout card is fine for DL Sky Clubs.

Plus she will get 12.5k miles for the trip. That's halfway to a low level domestic round trip ticket. A few more trips and she'll be there. Definitely worthwhile to have a SkyMiles account.