Give her info on a women’s center. This one helped me a lot: https://thewomenscenter.org/
Safe word & find a friend who could provide a safe house for her and the kids—someplace out of the area where BF wouldn’t think to look or be likely to run into her—and let her know where the house key is hidden should she need it. Your place is too obvious, but you could hide them until someone can pick them up. Presumably he can identify your car.
If the kids are in day care, can you call/approach her at work? Encourage her to talk to a counselor if she has an EAP, even if it’s only three phone calls. Can you take food to her at work so she can save what she would eat for the kids? The gift card is a great idea; she could pick up a few extras at the grocery store a little at a time so he wouldn’t notice unless he scans the receipts. Maybe just give her a bit of cash every week. Does she have her own car she could use to get away while he’s at work or does he work from home?
Have you talked to other friends and people in her family to get a better idea of what’s going on? Can you just drop in saying you were in the neighborhood and wanted to say hi, maybe with a bottle of wine? If he won’t let you in the house, that’s a red flag.
I’m so sorry. It’s heartbreaking and frustrating to see a friend be abused. Build up her courage and self esteem as much as you can, let her know you (and as many others from friends and family) will be there for her if she’s ever ready to leave.
I hope she’s just busy and stressed because of kids and funds and he’s too proud to accept charity, but I doubt it.